IP packets have wrong source address

I have two 4G modems connecting to a network and sending and receiving data. There is a problem though, as the modems keep getting kicked off the network because the source IP address of the packets is incorrect. It seems that packets that originate from datacard 1 have the source IP address of datacard 2. The mobile network checks the source IP address of the packet against the IP address of the connected modem, sees there is a mismatch and kicks that modem off the network. I need to rectify this problem and have both modems send IP packets with the correct source IP address. However, I'm very inexperienced in this kind of thing, and I would very much appreciate some advice for starting points. I believe that the IP tables and IP route have been checked and that hasn't resolved this problem.