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!
those “security filters” are the worst, a few years ago vodafone with their “rete sicura” was blocking githubcontent and it was a nightmare to have them disable the service for me, the operator was like “but this is a free premium service that protects you”