Hello, John. First, I'm really sorry about the connection trouble with your BRCK.
MTN should work in Cameroon and I see that you set 3G as the first preference. Could you try your BRCK with the settings in the links below:
MTN Settings 1
MTN Settings 2
Also, you can set the SIM settings from the Local Dashboard (more info on this link).
Click on Configure on the SIM tab and you should see places to input the APN information.
Click on Save and give the BRCK a few minutes to pick the settings.
The BRCK can still pick up SIM settings from the cloud. It needs to be online to enable you to do this.