MAX-BR1-BF3D ATT 5G SIM card. Compatible?

I am trying to use an ATT 5G SIM card with my MAX BR1-BF3D running 8.1.3 firmware. All I get is scanning for IP address with LTE and R with blue triangle. I am using a T-mobile SIM card in slot B and it functions when I am in an area with T-mobile service. ATT cell service is good in the area that I am located. Is the BR1 compatible with a 5G network or is that the reason I can not connect? Thanks for any advice.
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The paint/logo on the SIM card is irrelevant… What is the actual plan associate to the SIM?

I had a similar issue with my MAX transit duo; TMobile SIM was working well in Slot 1 and ATT SIM in Slot 2 was continuously trying to connect. The solution was to change the APN setting for the ATT SIM to “broadband” versus using the Auto configuration - hope that helps.

It is a 5G SIM card, which is supposed to be 4g and 5g. They have discontinued their 3g plans.
Not sure if that answers your question or not.

Sorry I am so not tech savvy, Looked on dashboard but where do I find the APN setting?

From the dashboard click on the cellular wan to bring up the cellular properties, near the middle to bottom of the cellular properties page you can set the APN.