I have a softphone app on my smartphone that connects to a cloud based PBX. The PBX is a common Broadsoft platform. If I open the app at home, behind my OPSense firewall, I get errors that it cannot register to the PBX. If I turn off wifi or get on any other wifi network, it registers fine. I've been hunting the solution off and on for weeks. I was looking in the wrong direction since I assumed it had to do with some network topology nonsense on my end. The solution turned out to be super simple. I found it in the very last sentence of this article (https://tech.iprock.com/?p=20880).
"If you are using a Dynamic DNS address, make sure you go to System: Settings: Administration and put that dynamic DNS address in the "Alternate Hostnames" field."
That's all I did. I added my DynDNS hostname to that field and it was fixed.