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!
Modern Android versions can use DoH (DNS over HTTPS) which can not be intercepted. If you don’t have this option or are not sure how to configure it, you could use the Quad9 app to enable secure DNS. This way you can make sure it is not related to DNS. Frankly, I can’t imagine they are blocking the IPs of the DuckDNS servers directly.
I’ve tried it with F-Droid’s Rethink DNS already and everything is fine then.
Pretty sure they are doing DNS packet inspection. If the DNS is to duckdns (and no doubt other Dynamic DNS), then port 80 has their block page, and not sure what they are doing with 443, but it’s not accepted.
Boringly, it’s not being messed with right now. It’s done this before. Coming and going. It’s one of the things that stopped me working it out before.
Edit: I was wrong. It was just working again generally.Even with DoH DNS, I still see “Access denied” going to duckdns.org and my sites still don’t work. Direct IP works. Not sure how this block is working yet.