Metro PCS uses T-Mo towers now. T-Mo bought them for the frequencies they hold and to have a national brand mvno to own.
T-Mo compatibility is what you are looking for and Factory Unlocked Lumia 650 and 550 from the Microsoft Store support that 100% for LTE, but are missing AWS on the 3G radio for when she leaves an LTE area.
640 GoPhone is a perfect match if it is unlocked and you can find one at a good price.
B&H has an Unlocked 550 that matches what you need in a radio at a great price.
Microsoft Lumia 550 RM-1128 8GB Smartphone A00026398 B&H Photo
Their 650 is a bit more:
Microsoft Lumia 550 RM-1128 8GB Smartphone A00026398 B&H Photo
B&H gets Factory Unlocked phones meant for the Latin American markets and they have great radio compatibility for the US as well. Just no Lumia warranty in the US, but B&H is a solid retailer with great service and they give you a one year warranty. I think they are your best bet. Be aware they don't take orders or answer the phone from sundown Friday through Monday morning as a religious observance Free expedited shipping in most cases..