An effective way to knock down GSM on demand
Buy a 10 bucks GSM/3G/4G radio jammer.
Cheap jammers have a range of 5 meters.
You should switch it on just for the time of your tests.
That's the fastest and cheapest way.
I did it in the past and it works.
To monitor if the comm port works, if the modem is alive and connected, just issue an AT+CSQ? every 12 hours.