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 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.