It too me a while to work out why my Nextcloud stuff wasn’t working on my phone. It wasn’t until I went to http://duckdns.org on mobile data I saw the block. I had changed ISP from one with IPv6, which I had setup, to an ISP without it, and thought it might be that. But it was just coincidence.
I’ve written to O2 but I doubt they will change anything, so I’ll be changing network.
So heads up UK O2 self hosting people!
You can try DoH to see if it is working.
It’s port 443 so it won’t get redirected by their filters.
Android uses DoT so maybe that works. Assuming that they don’t block port 853.
Try the encrypted DNS option to see if they are blocking all DNS providers or just certain ones.
You can also setup your own encrypted DNS on a VPS if you’re feeling brave.
Yer, I used Rethink DNS in F-Droid to test this already. Basically it’s fine when the DNS is encrypted.