You need to be in a Band 12 active area. Check T-Mo's map
T-Mobile's 700 MHz spectrum map - Click on the items for more information
You have Android 6.0 yet? It's working with the 6.0 update. LTE Discovery will show you when it switches over to B12 LTE Discovery - Android Apps on Google PlaySupposedly I am in such an area. Now what?
If you are running 6.0 on an approved device in a band 12 area, you should be good to go..Supposedly I am in such an area. Now what?