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!
O2 didn’t until recently. EE does’t currently (wife’s network)