Thanks for letting me know. Unfortunately there's very little I can do about DNS issues. I use Mullvad (VPN) all the time as well and I've never had issues with it, I'm not using their DNS over HTTPS though.

I've seen that, for example, Quad9's default service with content filters (9.9.9.9) wouldn't resolve the address, but their unfiltered one (9.9.9.10) did. Cloudflare and Google's seemed to work fine. No idea why the issues are intermittent for you though.

The public IP of my server is fixed though, I've changed providers a couple times but I'm happy with the current one and I don't plan on changing it any time soon. If you can edit your device's hosts file you can add a hardcoded entry for the current IP.