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!
If it’s a dns block I’d highly recommend setting up your own recursive dns resolver. Something like pihole and unbound. That way you query the authoritative servers directly and your ISP can’t filter your content as effectively since they would be limited to incredibly ineffective IP based filtering.
Pretty sure they are working via DNS packet inspection.