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!
Telstra here in Australia seems to have this as well. Not sure about duckdns specifically, but last night I found out that they block a few monero mining pools. I emailed them about it, and apparently it’s based off of virustotal ratings. They wouldn’t turn it off, but they told me it’s “trivial to bypass” (their words), suggesting google or CloudFlares DNS, or a VPN