mander.xyz is a pretty small instance devoted to science and nature communities. It's relatively tiny compared to lemmy.ml and beehaw.org. In order for a community to appear in the smaller instance's community list it must first be "fetched". To do this, you have to enter the full url for the community you want to fetch into the community search bar.
So for example, I was trying to find !nfl@lemmy.ml and it just wasn't coming up in the mander.xyz community list. I tried entering "nfl", "!nfl@lemmy.ml" into the search bar. I even navigated to https://mander.xyz/c/nfl@lemmy.ml which was giving me a 404 at the time but is now working after the fetch.
So if you're on a smaller instance and aren't seeing communities from larger instances, try entering the url of the community as it would be hosted from the larger instance. Hit enter to do a fetch, then try typing the name of the community again to execute the search.
PS: I had to do the same trick again on mander.xyz to find this community.
I ran into this same frustrating problem. The whole "fetches in the background after you search" thing makes sense in a very "you are technically correct" kind of way, but it would be a much better user experience to just have a spinner and wait for the community/post to be fetched, rather than gaslight users.