Unlocking a carrier-locked AT&T Surface 3?

Hi everyone. I recently bought a Surface 3 LTE second-hand. The box has AT&T branding on it, and it had an AT&T SIM in it which will give me a signal reading on the device and will connect to the network and take me to a registration page if I try to any website. I have an activated Verizon nano-sim that I popped in but it refused to connect to the network. I also tried a T-Mobile SIM and similarly had no luck (interesting side-note, it said the network names for each were "Ver" and "T-M", I believe). When I run diagnostics on the Surface in Network Connections, it says "the mobile broadband device or SIM is locked". I typed in the default SIM pin for the T-Mo sim (1234) and it says it unlocked the SIM, but it still won't connect. So after all this, I'm assuming the device itself is locked.

I went to AT&T's unlock site to request an unlock and I typed in the IMEI. The website says the IMEI is unknown. I tried calling AT&T customer service and they basically told me if the IMEI doesn't work on the website there isn't much they can do.

Anyone else had this issue? Any ideas?