Change APN on Smartsense TM-CELL400-Z-3G

We own a few SmartSense 3G gateways (TM-CELL400-Z-3G) fka TempAlert, but just received an email today these will be going offline soon, due to the fact that Verizon is shutting down 3G coverage.

I can change the SIM-card in these devices to use a different network, but would need to update the APN accordingly. How can we update these settings?

I tried to change the APN from the settings but suddenly I encountered a problem. I have checked for the problem. I tried many steps for fixing it.