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!
I’ve been doing some investigating. It’s not just DNS. Termux doesn’t use the system DNS, it uses Google. But there is still a interference with SSL on 443 and a different page on port 80.
Edit: oh and the IP address is current with ping.