bkastel, not sure if this helps you, but your problem description actually matches an issue I had when first installing my OSPi. I believe the symptoms would probably apply to a lot of internal vs. external network issues.
If I’m inside my WAN, I access my OSPi via the internal IP (192.168…) given by the router’s DHCP (static routes, BTW). Outside my WAN, say, on cellular or whatever else, I use the port forwarding IP. The port-forwarded IP does not work on my home WiFi, nor does the internal work from outside (duh). My solution so far has been to create two controller instances in the mobile app–one for @Home, and the other for remote, both using the respective IP addresses that work in each case.
Again, not sure if this helps, but I posted just in case it might.