How to get good initial ratings.
The most useful mechanism to effect ratings and especially initial ratings is rating bias: You want those people to rate your story that will give it full stars, while you want people that wouldn't give you full stars to abstain from rating your story. As ratings vary, even the most mundane story might get a perfect score, as long as you can dissuade those not giving you full stars (called down-voters in the rest of this post) from voting.
Very effective but not that easy is writing an story synopsis specific enough. To keep down-voters away, the synopsis should contain everything some readers might not like about your story, so that those do not even start reading. (As practically no one rates a story after only reading the synopsis). Bonus points for not stating it as facts (facts are data and some reader might still give it a chance), but by style and example (emotions are hard to overcome): Don't state your character believes in might-makes-right, that might attract some down-voters expecting character-growth in your story. Make your synopsis sound like you as author cannot even understand why anyone could doubt the mental health of such a character.
Once they have started reading your story, getting rid of down-voters before they down-vote must ensure they do not drop your story with a down-vote as parting gift. Here it helps that humans won't easily down-vote if they think it is their own fault that they cannot enjoy the story. It is often even enough if they cannot tell why they dislike the story. Then they either refrain from voting. Or if they do a review, others see that the reasons they gave for down-voting make no sense and thus might feel like to correct that 'mistake'.
One reason for not enjoying a story that most readers will not want to admit (and thus immunize them against becoming down-voters), is that a story is too complicated to read. So in order to get rid of down-voters, you need some tricks to make the story excruciating to read except for those that rate it with full stars. Especially good tricks are those that require readers to have a high familiarity with your story. As those are very unlikely to be down-voters.
One easy trick in this regard is writing your story less like a web serial and more like a novel.
Unless you publish your whole story at once, you are doing a serial. Writing a serial means not expecting your readers to still remember every detail of the last chapter that was many days ago. This means you have to make characters recognizable. You have to do subtle reintroductions of side characters all the time so readers remember them. So doing a proper serial is quite some work. And it doesn't help your (initial) ratings. Because when writing in serial format, everyone can follow your story even without rereading old chapters all the time and without memorizing your whole cast. You are practically rearing down-voters.
If you instead writing in novel format and only publish it like a serial, your avid readers will still be able to follow you and give you good ratings. Everyone else that is not a proper fan will get bored out of their mind because they fail to follow the story, so drop it before having a chance to rate it. And even if they think about rating it, they will consider how unfair it is to rate it. After all, if they just invested some more time and concentrated a bit more, they might have understood more and thus enjoyed it more. And the best part is: Novels are considered a higher form of art than novels. So no one can blame you for writing this way. Your only fault is making it available for free here instead of publishing as proper book directly.
Though if you write too good a novel, it might still cater to people not reading it in one go and thus contain some redundancy, not driving off down-voters as effectively. Some way to increase it more to counteract this, can be the proper use of names. It is quite hard to remember both very common names and very exotic names, especially for those readers cheating on you by reading other stories in between. So either name all your characters with common names like John, Michael, David, ... or give them quite exotic names (Chinese family names are especially nice. If they cannot keep track of Mr. Li, Mr. Xi and Mr. Ling over a few weeks, they are obviously not proper fans).
Another nice trick is what I call name-cycling.
The excuse for this is quite easy: There are some indicators for bad writing. Once of them is word repetitions. As with any indicator they are not a problem itself, but hint towards which part of your text you might rewrite. But after generations of wanna-be journalists got pestered with this indicator and them not being good enough writers to do something against the cause, they started treating the symptoms and some generations of wanna-be journalists and sports reporters later, most people are kind of used to people not being referred to name, but by some random properties and to have the same character being referred to in a different way each time.
This is a very effective and easy trick to keep down-voters away. Just act like an average sports reporter and replace names with someone else every other time: "David looked [....]. The high-elf threw [....]. The ranger hit [....]". Grammatically and by every rule this would indicate three different people there. But due to people being used to bad writing in news (and other stories using this trick), some readers might not even notice it and read it like if you used proper writing and without misunderstandings, if and only if they know by hearth all your characters and all their defining properties.
It becomes especially effective if treating multiple characters this way at the same time or if using properties last mentioned some chapters ago. Even better if you even combine them in situations without helping context. Like having three parties (say the MC, some allies and some enemies) appearing in some chapter and moving around independently. Then have some one of the groups do something where it is not clear from context which of those group does it (like in an area with bad visibility, a group suddenly appears near the MC, both having the allies and the enemies appear makes sense but makes a big difference) and refer to that group by some property not mentioned since two chapters ago (like by what they are wearing). Most of the fans will still remember that and not realize there was even some trap in there.
But any potential down-voter will be too confused to keep following your story. (Or at least not remember all the details so they have even less chance of following with the next chapter). You might also lose some of the readers that would have given full star ratings, but remember that for getting good ratings it is more important to get rid of down-voters than to keep the proper followers.
Also keep in mind that the techniques here work best with initial ratings. Once your story gets more visibility (like when hitting the Trending page), some of those will lose their effectiveness, as they are readers out there that stubbornly insist ratings should be useful for readers to find stories they likely enjoy and not as some reward for authors. So some of them will still rate stories anyway. But even then there is a way: Just point out to your proper readers how you suddenly get bad ratings once you appear in any of the top lists. As they kept reading, they will not understand how anyone could not like your story (especially with the good initial ratings proving them that everyone feels the same), so with some hinting will easily concur how those down-voters are treating you unfairly. And there is nothing so good at rallying people like fighting unfairness.
I would add as another important tip to getting good early reviews: make sure that the first 5-10 chapters are meandering and nearly plotless, making it look like you had no idea where you were going when you started the story; people will get really bored and drop the story early, so the only people left will be the ones who will rate it really well. Then, throw in a six-chapter-long fight scene; anyone who actually makes it through the whole thing is going to follow you to the end, and everyone else will just say that maybe it was something about them when they read it, that it's their fault...