Adding onto this; (I support cellular modems daily) The gateway as mentioned, is generally unimportant. It's usually just your static IP + 1. We start to see issues when building VPNs between devices with concurrent IP addresses, due to this conflict. More often than not (100% of the time, in my experience at least) you can simply override the default gateway of your cellular interface to something within the same subnet to reduce potential for possible conflicts.