The iOS Snikket app notifications only work with a Snikket (or specially prepared Prosody) server. Sadly it uses a non-standard way to deliver these as iOS is a bit complicated to work with in that regard.
You could ask the iOS users to try: https://monal-im.org/ (Edit: btw the login issue with the SLRPNK ejabberd server was fixed last month)
At least notifications should work more reliable with Monal, although it still lacks some other features compared to the Snikket app (a rebranded Siskin). I am not sure what causes the OMEMO error though as I am not a iOS user.