Raspberry Pi Modems

I went through a similar process recently while building a solar powered Raspberry-Pi based GNSS receiver.

(like the OP) I originally looked at a very low power/speed GPIO modem, but it wasn't very stable and not really suitable for what I needed.

I ended up with a SIM7600G-H-M2 4G HAT on a Raspberry Pi 3b+: https://www.waveshare.com/wiki/SIM7600G-H-M2_4G_HAT

It's a bit pricer but very capable and well documented. There is a physical switch to enable power input for the modem over a separate USB to the data connection, so the power for transmission can bypass the Pi electronics.

After fiddling with the first modem for hours, this one was working in less than 5 minutes with RNDIS: https://www.waveshare.com/wiki/Raspberry_Pi_networked_via_RN...

I noticed during testing that the 4G connection would sometimes drop and require manual intervention to reset... So I added a systemd timer to test the connection and bring the interface down/up again whenever it disconnects. No problems since then, now with two months of uptime with ~200MB upload per day :)