MR6500 no unlock prompt

I have an AT&T Nighthawk MR6500 that I have been given the unlock code for. However, I am never prompted to enter this code anywhere when I put in a 3rd party SIM. There's no option in the web interface either.

I just get a "SIM Failure - Please ensure you are using a SIM provided by your network provider." message. How can I get to the unlock screen to enter my code?