I think that might be a bug, the !community@instance.com template is pretty standard and even works on the web version. i reccomend updating the app, checking the github repository and checking the settings. But for me it works on the jerboa app. also you should probably try turning off your wifi connection on your phone and retrying, if that wasnt obvious enough. if those dont work try any chatrooms/communities memmy might have, they get resolved pretty quickly in my experience.