Android access IPv6 website only, on Mobile Network

I have a Nextcloud running on a Raspberry Pi 4. My ISP: Vodafone (Germany) didn't give me a unique public IPv4 address, so I had to use my IPv6 instead to set an AAAA record for my domain (cloud.mydomain.de). Then my nextcloud was accessible from other computers via cloud.mydomain.de But I was not able to access it from my mobile phone, both from Wifi or mobile network. After some online digging, I found that the DNS settings might be the reason for that, and I installed and app that can change my DNS settings. And changed my DNS to cloudflare. After doing this I was able to connect from my phone to nexcloud using Wifi, but not when using mobile network. This is strange, because I checked and my mobile network has IPv6 enabled, also when checking with https://www.whatismyip.com/ I have an IPv6 address. Also, I checked using mobile network that I can connect to an IPv6 only website, like http://ipv6.google.com/ So, why can't I connect to my website using mobile networks? Anyone experienced something similar? Any idea what I can check next?