I found out after our solar system was installed that the enphase inverter came with a cell modem for monitoring and remote management. Our installers didn't know how or even if it was possible to configure the system without one.
After I bought out our panels, I found the Enphase modem and disconnected it. It was a USB box connected to the monitoring unit, the monitoring unit has other networking options, and it's mine anyway.
Now you know to advise people to look into that question before the install/find an installer that can guarantee it. If the thing can't easily have cell function disabled (e.g. by pulling a readily accessible card), then advise people to stay away from enphase.
Enphase required the cell connection when I checked a couple years ago. Sol-Ark makes a solid hybrid inverter and allows offline operation.