[PROTIP] If you're on CM 12.1 and not able to get LTE data, try this

For anyone using CM 12.1 and not initially able to get LTE data, try deleting ALL of the pre-existing APNs so that the list of APNs are empty, and MANUALLY add in your carrier's 4G LTE APN.

Worked for me using MetroPCS 4G LTE.