We have implemented an GX450 which receives UDP commands from our cloud based server using verizon. Port forwarded a range 7000-7004. Our device on the protected lan side of the modem receives and reacts to these packets. In response to incoming UDP packets our protected device issues a response UDP packet to the static public IP. Using packet capture we can see the response packet on the lan network, but we cannot see the outgoing packet from the modem. For testing we created a timed outgoing UDP status packet. This packet also can be seen internally on the lan, but not externally on the cellular network. There are no outgoing firewall rules. We have success with incoming and outgoing TCP packets to multiple devices on the lan using port forwarding.
Not sure if anyone has experience a similar issue with UDP. Sierra has tried without success to duplicate the issue or propose anything more than a best guess and to do a firmware update.