For the scenario, you want to implement. I would like to know what type of IP address are you getting from the said SIM cards? If you ping the SIM card IP address on the other device, the other device is able to reach it can you do it vice versa.
To explain it more clearly kindly check the topology below:
Let say as an example 10.10.30.170 is the SIM card IP address from the AT&T Sim card and the other sim is having 10.10.30.150 IP. You need to check in your side if they have end to end communication through ping. Now if the sim card has end-to-end connectivity.
Configure the Modbus Slave like this:
Modbus Master configuration:
Note: The IP address you must enter is the WAN IP address of the SIM card of the Modbus Slave router.
I hope this helps