Cm175
SanDisk is a good brand. Standard size, not the itty bitty one? Can you try a different SD card?
What firmware does the information screen say that the camera has?
This is very, very unlikely, but if you don’t have an unlimited plan, you still have data and are not in Safety Mode. (Within the last few days, a Verizon 4G owner had a camera that wouldn’t send and it was because the grand baby had used all the data and the account was running in safety mode.)