Mlem does have search. Tap the middle of the top bar, where it says “Subscribed” or “All”. Then tap “Communities…”
I’m surprised people can’t discover this, considering it’s the same workflow as Apollo.
I tried it, put in the link to the community (one of the main Support ones) and nothing happened… tried with another, again nothing. I thought I was using the wrong link, went from the web app, searched for it, copied the link from the search results, went back to Mlemm, pasted it and again nothing…. :(