Same here. As I'm not up my workshop much these days I've cancelled the broadband contract and just use my phone as a hotspot for the Desktop PC and firestick on the TV. I have 12Gb for £6. I wouldn't know how to connect a SIM card operated device to my cctv setup though.
Some cctv systems have built in or plug in modules to send texts to a phone.
Don’t know how they would send live pictures unless they rely on the text switching the “send data” on either by the owner responding to text or possibly a timed interval?