Hi, i see the Server URL (Settings > Server) is used for clients to see the web interface, is there no way of setting a different URL for internal and external clients? or does this rely on a DNS name that will change depending on the DNS server the PC can see? (such as backup.domain.com)
Use a DNS override if using PfSense to point to the same domain internally. That way your SSL cert still applies. Not sure if this applies to your set up.
The main server is not SSL enabled, i am aware about the internal / external DNS setup, i was kinda suprised it didnt have an internal and external IP address for web access though.
I’m not sure how a client would know if it was internal or external. I have laptops that travel in and out that can resolve the same DNS name which made things trivial. Probably a good idea to set up a reverse proxy for SSL anyway if connecting remotely. I can post how I did it with Debian 12 if that helps.
Good luck 
The client software knowns, it will tell you if it found the server on the LAN or Internet.
I think I enabled 'Clients try to connect via Internet/active client: ’ so they always use the internet settings. I checked a local machine and it shows (Internet: Yes).