secondary signal

T-Mobile deployed 4G LTE on B71 in 2017 on the 600 MHz band and then in 2019 began deployment of 5G on the 71 frequency band. Given the bands you refer to T-Mobile does leverage those and it is highly likely they have been upgrading equipment on the towers. If you can identify the PCI, physical cell identifier, of the 4G LTE and 5G NR cellular signals you can use CellMapper.net to locate where the tower is located and how far it is from your location. With the tower information and the cellular metrics you can get a better expectation for service delivery. Work on the tower equipment probably does account for the LTE change. If they took the original 71 band and have now deployed 5G on it then the original equipment with LTE delivery is probably retired therefore the B66 LTE frequency is what is left. Depending upon their objectives and planned cell deployment for that tower coverage things could possibly change some more. If the B66 is only at 1 bar the uploads could be rather slow. If the signal quality of the n71 is good then with 3 bars the downloads on the 5G could be fairly respectable. Signal quality, signal strength, and signal to noise ratio will all be important factors in service delivery. 

I would speculate that there will be a bit of disruption for maybe weeks if they are doing upgrades to the equipment in that location. Contacting T-Mobile support to try to get some answers may take a bit of time but would probably be necessary to have more clarity.