[Solved] Opening home server to the Internet via IPv6

I've been wanting to set up a small game server on my home network for myself and a few friends lately. Nothing I haven't done before - except the part where I open it up to the internet for people outside of my home network to play on.

So I tried setting up a small web server to test out the port forwarding functionality of my router. Darkhttpd, running on a spare Raspberry Pi, works fine on the local network. After digging through the web interface, I find out that using IPv4 isn't an option because of how my ISP tunnels network traffic (sth sth Dual-Stack Lite)—fine by me, in 2024 we should be using IPv6 anyway. So I go and open up port 80 in my router's web interface.

This is where the problem begins. Everything looks fine, but I don't have ready access to a network outside of my own to check if the port is actually accessible from the internet. An online IPv6 open port checker I found tells me the ports are visible and that my ISP isn't blocking anything. Trying to bind a domain that I had lying around to my IP address, however, has resulted in failure.

I have no idea how to debug this. I'm pretty sure there's some issue on the DNS Server end, but I can't even tell if the rest of what I'm trying to do is working. And if it is, I have no idea of how to go about fixing the DNS thing.

Update: I got a friend to test it, and the web page is accessible from the internet. Problem lies with the DNS server

Update 2: After contacting my friend again for a sanity check, it seems that the DNS server works fine and my test website can indeed be reached through my domain—it's just that I can't reach it.

Update 3: After poking at various DNS servers, it appears that the Mullvad DNS servers which I use don't regularly update their records. I've now switched to Cloudflare. My router similarly implements some caching solution that, after much tinkering, I was unable to flush. For the time being I've just decided to fuck doing this properly and directly edit my /etc/resolv.conf with the Cloudflare DNS servers. If I ever manage to get this working properly, I will add a final update, but for the time being, I will consider it solved.