Maybe the SIM cards are faulty, a SIM card is also a piece of hardware with microelectronic. I had several support cases as FP2 angel, where the SIM card was to old (> 4years) or damaged (proven by changing with another new SIM card).
Or sometimes the new nano SIM are too loose inside the micro SIM adapter (rim).
Edit: I had one case this month where oxidated antenna contacts seemed the source of weired network behavior and reboots. (Bad radio performance caused by oxidation and scratchmarks on display module backside contact pads)