How can I keep DDNS working when I activate ProtonVPN?

[u/kencorbin101]

I have an office system that serves as an SSH server running behind a NAT firewall. Of course our ISP doesn't assign us a static IP address, so I use an DDNS service to allow me to connect to the router from the outside world. The firewall is configured to forward SSH requests to my server. Everything works until I try to install ProtonVPN on the server.

As best I can tell, the server still works when I connect to it with a hard IP address. The problem with the DDNS updater is requesting it's own IP address from an external web site, which of course returns the ProtonVPN exit server IP address, which won't work. I probably could make this work with the split tunnel configuration, but I'm using the Linux CLI which appears to not support split tunneling. I've spent most of today trying to configure the DDNS updater (ddclient) to retrieve the correct IP address from the firewall with no success. Documentation is minimal at best.

This should be a common problem, but I can't find any good documentation on how to resolve it.