The nightmare of all writers. These tips may sound like hypocrisy, but I don't think one writer needs all. Each person is different and may profit form X but not from Y. Though, I believe one is more effective but can be mixed with another tactic.

A Writer's Block may be your brain demanding a break. Be it from the story you write, the fandom or writing in general, you need to decipher the blockage first.

It either helps to write something different to what is blocked, or not writing at all what you read.

Maybe do something else for a bit. Be it watching YouTube, hearing music, reading (Not necessarily Fanfiction), drawing or making Covers on Canva! Maybe play a game or take a walk.

All in all, to avoid Writer's Block, avoid Burn out. Don't do the same thing constantly.

For the fandom and story specific Burnout or Writer's Block, it helps to have multiple WIP's, or work in progress, going. But not too many. Too many causes indecisiveness and an inability to finish something. You only start new projects but never finish something. At some point you're overwhelmed to write anything.

I cap out at 3 or 4 projects at a time. Two are the same Fandom, but different in other ways. One is a medieval AU, the other an anime rewrite. I also work on a Translation for a different fandom  and one One Shot at a time. They are different enough to allow me to switch around to not burn out. But different in nature to keep the writing part itself fresh.

All in all, everyone writer is different. Try out things, not everything works on everyone.