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.
For me it was the other way around. I often had to use the complete url to the community including https:// for the discovery to work.