I have had this exact issue. I had an AT&T and Verizon nano-SIM. Tested both in another MX67C and they both worked, but they both would not work in the spare unit I had. Contacted support, and they told me that the device was not detecting anything, however I could clearly see the 3 metal marks making contact on the nano-SIM cards.
Ended up having to RMA the unit, and the new unit worked just fine.
I'm going to bet this is what your running into. I also had another unit where I could hear something rattle inside, but the built-in cellular did work, but I had to toggle it on/off a few times to finally get it to register.