GX450 Port Forward issues

I am using a GX450 with AT&T sim card in my vehicle. I am having issues port forwarding to UDP port 9742. I have gotten limited support from my vendor and since they are happy port forward is set up correctly it must be AT&T’s issue. Call AT&T and its the modems issue. AT&T wants to charge me 100.00 a month for a sim card that can be assigned a LAN address. Why does the modem allow you to create a LAN address under the reserve DHCP settings? For a broadband connection what are all the steps I need to take/follow within acemanager’s current version 4.9.3 to verify I have port forwarding set up correctly? do I really need a static LAN address assigned by AT&T if the modem allows you to create one? is there a work around foe the WAN/LAN issues where it appears the modem doesnt know where to send the UDP packets to?