Whew, what a battle! But, I think I have won.
Problem: The extension I was using was Chan_SIP, pretty sure PJSIP (+ more than 1 Contact) is a requirement.
Solution1: Convert extension from Chan_SIP to PJSIP using button in UI. (PJSIP was already listening UDP 5062)
Result1: Extension would never register.
Solution2: Delete extension and recreate as PJSIP
Result2: Extension now registers on wifi, will not register on mobile data.
Solution3: Enable PJSIP on tcp 5062 and modify iptables accordingly
Result3: Now working 100% (including my lock screen issue mentioned previously). Works on mobile data, wifi, and when screen is locked/sleeping. Strange, I know. I have not re-enabled face, fingerprint or PIN unlock. Still just a simple swipe up to open the phone.
I think the tutorial needs to mention PJSIP as a requirement, or if it does, I missed it. And no, I didn't read *any* of Clearly's documentation, maybe it's mentioned there.
Do I dare now move on to TLS? LoL