COVID-19 is disrupting daily life in unprecedented ways. T-Mobile has already opted to close many of its retail locations, and now it wants to speed up your data. People may be relying on their mobile connections more than ever, so T-Mobile is renting some unused band 71 spectrum around the US.
T-Mobile is the only US carrier using band 71, which is a 35MHz chunk of spectrum around 600MHz. However, it doesn't have access to all of those airwaves—it only has 10MHz in some markets because companies like Comcast and Dish are sitting on the licenses. That can be a problem as 600MHz doesn't support much bandwidth to start with, but it's good at covering large areas. If it gets congested, you can end up with a measly couple of megabits down. The 600MHz situation is particularly problematic for T-Mobile right now because it's running both 4G and 5G on that frequency. That means some markets have as little as 5MHz allocated to each network.
T-Mobile's plan to rent unused band 71 spectrum will probably focus on 4G; for example, creating a 20MHz (the max available on LTE phones) and a 10MHz channel. The network would connect 4G devices to whichever one was less crowded, and still have 5MHz for 5G. However, T-Mobile could choose to create a larger 15MHz channel for 5G (with 20MHz for 4G), but that wouldn't be useful to as many people because 5G devices are still rare.
Renting this spectrum from Comcast, Dish, and myriad investment firms could help keep data speeds tolerable during this trying time. Although, it will also give us a glimpse of the mobile network performance we should have had all along. This experience won't be permanent until the FCC stops letting companies hoard spectrum.