This is just a proxy bid, as quite some tech info missing regarding the devices involved. Sounds like some type of GSM-Gateway/SIM-Server/-SIM-Bank network, using TP-Links.
Relevant Skills and Experience
Look at my feedbacks, pls. Your problem might be the VPN-Setup or routing. But for further evaluation, I need tech info regarding the hardware used, first, to see, how it _should_ work.
Proposed Milestones
$222 USD - completion
Is it possible to ssh into the TP-Links, from the web ?