As someone trying small instance I have to explore other instances using good old browser and then try to open it in Boost. For some reason sometimes it just doesn’t work, even for communities that are in lemmy.world! Ones I have tried and failed are:
https://lemmy.film/c/moviesandtv
https://lemmy.world/c/linux_gaming
https://programming.dev/c/programmer_humor
And some others I may have forgotten. For comparison, if you click https://lemmy.ml/c/asklemmy it should correctly open in Boost instead of browser.
It’s tricky and something that largely sorts itself out over time as an instance matures. It was a lot of leg work early on manually searching up communities with lemdro.id but has been smooth sailing the last few months!