BLU life one x2 - sim2 (4G) disables sim1 (2G)

I have an AT&T go phone sim (for voice and text), and a freedompop sim (for data).

If I put either one of them into the BLU phone, it works.
If I put both of them in, then the AT&T sim stops working -- signal strength goes to zero bar, and calling the number doesn't ring the phone. If I leave the sims in and disable the freedompop sim in settings, then the AT&T sim works again.

I found that if I put in just one sim, the BLU phone let me choose whether to use 2G/3G/4G/LTE on the AT&T sim. If I put in both sims, suddendly the AT&T sim is 2G only and cannot be changed. I think the BLU phone may only support LTE on one sim while the other sim can only be 2G. Can anyone confirm or deny this?

Since AT&T turned off 2G service, this is a problem.

 

I found the answer: in this phone, only one sim slot support 3G/4G, the other slot (meant for voice calls) can only support 2G. This is not mentioned in their product description or user manual. When I contacted BLU to ask about this limitation, they avoided the question and sent me the answer to a different question.