Some thoughts -- as I understand it, Vio needs to be at the same voltage as the logic voltage. From the tutorial, the section on pinouts: "A 5V micro (like Arduino) should have it be 5V, a 3V logic micro should set it to 3V".
I use an Adafruit Feather (#2771), which is also 3V logic, to control my FONA 3G. I tie the 3V output on the Feather to the Vio pinout on the FONA.
Something else that might help -- if you have decided to only use an active antenna, you also need to solder that "bias" bridge near the connector, described in the tutorial under "Antenna Ports".
Of course, if you decide to only use a passive antenna, you don't solder that bridge. But from various articles I've read if you do use a passive antenna, there will be a several minute delay before the GPS will begin working the first time it's powered up. Has something to do with "data tables" being downloaded, see link below:
https://stackoverflow.com/questions/342 ... gps-module
This may/may not matter -- the only other thought I have is since you are powering both boards with the USB is to remove the wires connecting their 5V pinouts together. Again, may not matter, but it doesn't seem needed and keeps things simpler.
Let me know what happens, I like tinkering with my FONA 3G, and also have a SIM7000G breakout board, I find this stuff interesting.
