@Jennifer W. I have not been in a position to need to use this type of system to this point but I do have a suggestion for you, not sure if you listen to the bigger pockets podcasts but they do an outstanding job of not taking sponsors just for money, but selecting sponsors that can actually provide something of value to listeners and members of this website. I am not positive but I do believe they have only every advertised one company for the type of security system you want. take a look at SimpliSafe, if you click on show notes for podcast #193, and many other podcasts. there is also a discount code on the link
I've been testing cameras with video sent over cellular.
Right now I'm testing a Xiaomi Yi dome camera and a generic RTSP IP camera.
The camera is connected via wifi to a TP-Link router with a LTE air card plugged into the USB port.
Both cameras work fine for monitoring. My goal is to have the video sent to a cloud NVR 24/7. The challenge I'm running into is AT&T is AT&T doesn't open any ports to make it possible.
I heard it's possible to get a static IP on Verizon with ports unblocked, so I'll test that soon.