As silly as this sounds, I spent hours troubleshooting this issue. Double checking configs, firewall setup, reinstalling the server, etc etc etc. I ultimately resolved the problem by... reinstalling the client software.
That's right, my iOS app wouldn't connect to the server. Maybe some kind of caching from a previous error on my end? Anyways, I deleted the app, reinstalled it, and suddenly could connect without any problems. If you've tried everything else, it may be worth a shot.
Same issue here. Sometimes, on my iPad only, the server will appear in the friends tab and let me connect to the server.
Same issue here. I did manage to get my iPad to connect to the server via the friends tab, despite no other device actually being able to connect to the server. See potentially related bugs #711 and #635
This is still an issue with the current version of Minecraft, at least when using a ps4 controller.