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 they take long or don’t resolve it, try the live support chat. I used the chat inside their app to request it and it was unlocked pretty much instantly.
I tried their help chat bot and it just said to install their app after a few exchanges. My phone is de-googled, so good chance their app won’t work. Plus, I avoid installing closed apps as it is unhygienic. Each one seams to demand access to contacts and location and won’t work if you deny it. Dystopian present, let alone future.