I looked and it appears that the S6 supports 2G, 3G and 4G service. Being an older model device many of us have heard about the 3G shutdowns and we have seen users with questions related but given the specs of the S6 it's not likely to be affected by any 3G shutdowns.
With the concern you've mentioned about the Ex, did the devices stop working at the same time as their departure or was it just close to that time and so you suspect something? Anything they may have done would have happened that day, it wouldn't be something they could do remotely or after being out of physical contact with the devices. This brings up mustang7757's question of, is this your plan or are you sharing a plan, with someone else as the primary account holder? If the plan is shared, I would suggest you contact the primary, verify that your devices are still active on the plan, and follow that up with B. Diddy's advice to contact the carrier. Or just the carrier if you are the primary.
You mentioned that these are older devices and at least one is a backup device, but have you confirmed the SIM card is inserted in the devices. You don't mention checking the SIM cards for any reason, but again if foul play is suspected, just checking if they have SIM cards might be prudent.
If the plan is good, the SIMs are correctly installed, the best advice is going to be contacting the carrier since this is affecting your service, or the lack of.
Let us know what you find.