Is there a way to determine what mobile network frequency band is in use? I use a Nexus 5X on T-Mobile's network in the United States. Is there a way to tell what frequency band is in use when using LTE? A Sony Xperia Z3 compact also on T-Mobile is unable to use LTE in many situations where the Nexus 5X can use LTE. I suspect it may be that the Nexus 5X has support for frequencies that are not supported by the Z3 compact, but I would like to see what is in use.
The LTE Discovery app can give you this information.

Incidentally, your specific situation is likely a result of the 5X supporting band 12, and the Z3 lacking it. This tends to be relevant in situations where reception differences are significant (one device with service, one without) because band 12 is T-Mobile's only low-frequency LTE band, meaning that it penetrates buildings better and will cover wider areas more effectively.