configure RV50 in UDP multiple unicast


I have a project that will use a punch of the RV50 for the communication, our device will exchange some message using the modems, i read the “User Guide for AirLink RV50” to find the best way to do it, and it was the UDP multiple unicast, such that one of my devices will use the ethernet to get an internet access so it can reach the modem,
for the time being the device which is connected to the modem is able to transmit a message to the device with ethernet connection but MY PROBLEM IS IN THE REVERSED DIRECTION. in other meaning what are the modem configuration which allow the modem to receive the the incoming packets from the external world.
P.S. the SIM card has a static IP address and the ethernet device as well and my devices are connected to the RV50 over the serial port.
also when i ping the cell modem i won’t get a reply while if i ping the ethernet device, i will get a reply.

Please help, i reach the deadline for the project delivery.

Mohammed Fawzy