Ralph,
I will be contacting AT&T on Monday to update myself on their Static IP implementation. I do not want to speak unfairly about AT&T regarding how they are now handling Static IP provisioning.
There was a time...when if you had a Static IP at the base on AT&T, you could not access the base from an AT&T modem at the rover. You could use a Verizon at the rover with no problem. This has to do with something called "provisioning", which I am not fully knowledgeable about.
My solution was to use Verizon for the Static IP at the base. You can then access the base from any cellular carrier.
Of note is that with Verizon, you must set this up on a business account, and they will charge you a $500 one time fee to provide you with up to 10 Static IP addresses. You only need the Static IP for the base. If you had 10 crews, this would be a trivial cost. For a single user, that is about the cost of 5 awesome bottles of your favorite.
And please do not forget....I love "Netgear" brand modems such as the model 791L, and I really do not like the Novatel modems such as the model 6620. Novatel modems may be fine, if only used with Windows, but have caused more intermittent connection problems with our gear than my patience can tolerate.