How to let services in gluetun connect to LAN IP address?

[fooness]

I have a (sub)domain/FQDN with a public DNS A-record pointing to a LAN IP address, for example:

service.example.org -> 10.0.0.123

I want to connect one service (which is part of the gluetun network) to a self-hosted ntfy instance, which does not work by defining an internal hostname, like http://ntfy.internal:80, and instead I’d need to put: https://service.example.org

The application displays the following error message:

Unable to send test message: Resource temporarily unavailable (service.example.org:443)

Unfortunately, gluetun cannot resolve and/or connect to the LAN IP address the URL is pointing at:

$ nslookup service.example.org
Server:         127.0.0.1
Address:        127.0.0.1:53

** server can't find service.example.org: REFUSED

Non-authoritative answer:

nslookup does work when defining an upstream DNS: nslookup service.example.org one.one.one.one.

I tried searching the repo and the wiki, skimmed through gluetun’s DNS and firewall options, but I’m still not entirely sure on how to achieve this without maybe getting rid of the upstream DOT provider (Cloudflare) and potentially risking DNS leaks.

Might anyone be able to help, please?