T-Mobile has made changes to many of their tower locations in an effort to upgrade their existing service. As opposed to just adding frequency space, they have removed certain frequencies from towers to make space for these upgrades. Unfortunately that means a downgrade of service to many users.
This change has not affected all users, so if you have not experienced a reduction in speed this does not apply to you and may not for some time.
Unfortunately a solution to this problem is forthcoming as there is no readily available LTE module with access to the new frequency, but we are working on resolving this as quickly as possible. We will continue to communicate progress as new information/technology becomes available. We expect viable solutions to be available to us by the end of April 2020.
In the meantime, if you continue to have problems and are utilizing the Netgear LB1120, ReadyNet LTE520, or Mofi 4500 SIM4 model modems, you may consider a change to the Netgear MR1100/M1 Nighthawk modem which has access to the B66 frequency band and may offer better throughput.