I also self-host nextcloud with a proper url and use the android client on graphene.
For some reason I never nailed down, the app was particularly picky to my hairpin routing. Ij that I could access it from outside the LAN, but not inside the LAN.
I fiddled with a bunch of stuff on the server, the router and phone ans then it just started working and has for years,
The other issue I had, was the NC app being picky with self signed SSL certs. I have had zero issues with that since I started using Traefik and LetEnceypt.
There is ligh at the end of the tunnel, I've had my setup stable for years.
