Here’s what I think happened: Everything you see is something you did.

The subdomain you host points to your IP … or at least it did. Your ADSL IP address has changed and your domain is now pointing at someone else’s IP as their ADSL has claimed it.

Ping the subdomain and see the IP address. Now go to https://whatismyipaddress.com/ and see what your IP address is. They’re different now, right?

Wildcard certificates and HAProxy on pfSense is how I do it.

Loads of us run Ubuntu VMs on Proxmox.