From my reading if this and the last post you linked above. Perhaps your problem lies here.
Would not a static Mac address assignment
Be a better solution for your WWAN card.
The it would use DHCP and reconnect.
Your WAN / Global ip would change.
But so does mine.
Your configuration / setup is probably tied more so to your WAN / WWAN mac address.
So my thought is a fixed MAC addresses
Is probably the correct path.