Help! M6 Hardware or Setting Problem?

Hi!

I recently got an unlocked M6 via Amazon Renewed (MR6110). I have a T-mobile BYOD plan, and I checked the IMEI number with the t-mobile website, which shows no issues. When I put in my sim card, it just says "searching." It shows the device info and the number and all that, but it can't find the network. At first, the M6 was saying it's connected to the Verizon network, but it wouldn't connect to T-mobile at all. After calling support at t-mobile, I added the APN settings to match my current router. It's weird: my current router works just fine with the sim card, t-mobile says that there are no issues on their end. Any ideas about what I'm doing wrong or how to troubleshoot this? I do know that the firmware is out of date on this router... but I can't update it without internet....

Any ideas??? T-Mobile suggested that it's a hardware problem, but no one can describe WHY the hardware would be bad... and I don't want to try to return and get another router unless I have some hope that a new one might actually work! Thanks for your help!