Finally got to the bottom of this.
Suspect certain ISPs block remote terminated connections of certain protocols (HTTP and SSH in this case) for certain IP ranges. The public static IP block the mobile ISP assigned in the last instance seemed to have been blocked by at least T-mobile and AT&T.
GUI accessibility tests with IP assigned in the last instance.
* Verizon. OK
* Comcast. OK
* T-Mobile. Fail
* AT&T. Fail
Known good SIM mentioned above succeeded in all instances.
ISP allocated a new IP address in a different subnet, after which the GUI was accessible on the public static IP on at least Verizon, Comcast and AT&T.
Hope this helps someone.
Kind regards.