Hi Mark,
Drop me a mail at dj0qn (at) darc.de and I will send you my networking and 3G/4G checklists.
This may give you a tip or two on setting this up.
It looks like you solved the main problem, which is the normal "double NAT" problem using 4G
at your remote station. It looks like you have a "real" external IP number in your case.
73,
Mitch DJ0QN / K7DX