Server Version#: 1.22.1.4228
Player Version#: not relevant
My issue appears to be identical to the issue described here:
On the server (Ubuntu 20.04) connected to a VPN with a split tunnel set up (my actual external IP is used by the server) and a static port forward, the remote access check is appears good for anywhere between 5 and 20 seconds, then switches to “not available outside your network.”
If create a route for SSDP traffic and enable UPnP, I get the same results.