GSM alarm switch off with 3G?

[Trundlebug]

How can I find out if the gsm alarm in my boat will continue to work or not when 3G is switched off?

 It has a Tesco mobile PAYG SIM, (using the O2 network), the old kind which has no monthly payment and the credit doesn’t expire.

For some reason this question is completely ignored in O2’s guff on the subject of 3G switch off.

 I know I’m not the only one using a SIM in something other than a phone. Alarms, vehicle trackers, IOT devices used in homes and industry and all sorts of equipment have SIM cards in. Yet it’s not mentioned anywhere, even down in the sub submenus of the “help”, instead they focus on iPhone12 and later 🙄.

Any help would be greatly appreciated.

[MI5]

@Trundlebug 

It all depends if the device the sim is in is capable of dropping to 2g for the connection.

If so, it will remain connected, but if not, it's useless.

If it's a fairly new device it should be capable of 4g too, so you may need to get a new sim to allow that connection.