Hey Gus, for testing in the past I have used an adjustable RF attenuator (similar to this) in line with my Particle module and my antenna. It's great for simulating devices on the "edge" of signal, or killing it altogether.
I bought mine on eBay for pretty cheap along with some uFL to SMA adapters. There also appears to be some digital attenuators out there, but I have not tried those out yet.
30dB has been enough for me to kill my connection, but it may vary based on how strong your baseline is. I will note I have been surprised at how performant the modem is when I increase the attenuation gradually vs. all at once, or before the modem begins to connect.
A couple of other notes:
You may consider picking up some SMA static attenuators in addition in case you need more than 30dB. They should be inexpensive.
That attenuator may have the ability to do a full disconnect of the RF path
Love to hear your experience here once you get your setup together!