Can my Huawei P20 Pro be upgraded to LTE?

I'm using my Huawei P20 Pro with Straight Talk (AT&T towers).  The phone has dual SIM slots, only one is in use.  It appears the phone may be LTE capable, but is not currently compatible.  Is it possible to put a second SIM card in the phone that will make it LTE compatible?  This phone is only 2 years old and I paid a lot of money for it.  I really don't want to buy a new phone.  Can someone help?