Hi! Did you manage to solve this issue? I have a similar requirement. I have a set of battery powered sensors on a remote cabin with very limited power supply during the winter. I think I can manage to continuously power an Arduino Mega gateway, but not a 4G router to send the sensor data to my database at home. I think the solution is to have the gateway buffering up sensor data, and then send it once a day or something. (After first powering up the 4G router through a relay).
I guess I have to hack into the Ethernet gateway code, like you mentioned. But what would be the best place to get to this information. What file/function I should begin digging into? Please advise, if you know the answer!