Joshua Goldman/CNET

T-Mobile completed its Sprint merger less than three weeks ago amid a pandemic, but the carrier isn't letting the coronavirus put its plans on hold. After announcing on April 1 the first use of its newly acquired midband spectrum in Philadelphia, T-Mobile now plans to redeploy airwaves in parts of New York in May as part of the latest expansion of its 5G network.

In turning on the 2.5GHz spectrum in New York, T-Mobile will have a first city to showcase its 5G vision. The carrier turned on higher-frequency millimeter-wave 5G in parts of the city last summer before adding its low-band, wider-reaching 5G network nationwide in December.

On Tuesday, T-Mobile will also expand the low-band footprint to include the Detroit, St. Louis and Columbus areas.

Whereas the millimeter-wave network has fast download speeds but severely limited outdoor coverage, and the low-band much better coverage but not noticeably improved speeds compared with 4G LTE, the midband spectrum allows for a combination of both with significantly faster speeds compared with 4G for service that works both indoors and outside.

Karri Kuoppamaki, T-Mobile's vice president of radio network technology and strategy, said the carrier is seeing download speeds of nearly 600Mbps in Philadelphia in early testing of its midband 5G deployment.

Read more: T-Mobile has merged with Sprint. Here's what it means for your devices

Most of T-Mobile's 5G devices -- last year's OnePlus 7T Pro 5G McLaren and Samsung Galaxy Note 10 Plus 5G as well as this year's line of Samsung Galaxy S20 5G phones, LG's V60 ThinQ 5G and the forthcoming OnePlus 8 -- will be able to tap into T-Mobile's midband 5G network as it rolls out.

Sprint's Galaxy S20 5G phones will also be able to start using the new T-Mobile's 5G networks later this month, with the support for T-Mobile's low-band 5G being enabled on the Sprint devices with a recent, already released software update.

Those with Samsung's Galaxy S20 Ultra 5G or Plus 5G from Sprint or T-Mobile will be able to take advantage of all three flavors of T-Mobile's 5G network when available. Those with the other devices will be able to use the low-band and midband 5G but not the millimeter-wave.

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An end for Sprint 5G and its early 5G phones

Although Sprint's Galaxy S20 phones will be brought along for the network upgrade, older Sprint 5G phones from 2019 will not be getting updates to enable access to T-Mobile's 5G network. Instead, those devices will remain capable of tapping into Sprint 5G in the markets where it's available for as long as it's active.

To make the midband 5G spectrum work better with its other 5G networks, T-Mobile is taking the already deployed Sprint 5G and refarming it, a process that shuts down the existing network and redeploys the spectrum.

This process will take place in New York as well as, over time, in the other eight markets (plus parts of Miami) where Sprint's 5G service is currently live. The carrier says that it will turn off Sprint's 5G network in New York City shortly before turning on its redeployed version.

As with other Sprint 4G LTE phones, these 5G devices from last year will still be able to use Sprint's 4G LTE service and take advantage of T-Mobile's 4G LTE when Sprint's network isn't available.

Sprint users have been able to roam on T-Mobile's 4G LTE network since March as part of an agreement to help keep users connected during the coronavirus pandemic. With the merger between the companies now finalized, that roaming will continue.

Part of the reason behind the move is that Sprint's older 5G devices lack the necessary modem to connect to T-Mobile's 600MHz low-band 5G network, a chip that wasn't made available until late last year.

T-Mobile told CNET last summer that it would allow the Galaxy S10 5G sold by Sprint to work with T-Mobile's millimeter-wave network once the carriers' merger was cleared and that its version of the S10 5G would be able to work with the redeployed Sprint midband spectrum.

That will no longer be the case for either device. The S10 5G lacks the capability to tap into the LTE anchor band for 2.5GHz 5G that T-Mobile is using, a band combination that is different from what Sprint has done. This combination is need so that the transition won't impact Sprint users who are connected to the midband spectrum for 4G LTE. (The T-Mobile S10 5G will, however, still be able to use its millimeter-wave network where available.)

T-Mobile says it's planning to alert people who bought Sprint 5G phones about the need for new devices. It will have a number of offers to encourage these owners to upgrade to a Galaxy S20, with the offer varying slightly depending on how much they're paying per month.

Those who currently own, lease or make payments on a 7 Pro 5G (256GB), S10 5G (256GB), V50 ThinQ (128GB) and are paying less than $10/mo. will get a Samsung Galaxy S20 5G for $0/mo. after $41.67/mo. credit with a new 18-month lease.

Those who currently lease or make payments on a 7 Pro 5G (256GB), S10 5G (256GB), V50 ThinQ (128GB) and are paying more than $10/mo. will get a Samsung Galaxy S20 5G for $10/mo. after $31.67/mo. credit with a new 18-month lease.

Those who purchased an HTC 5G Hub on an installment plan will get a credit of $12.50/mo. for the remainder of the term. If they purchased the hotspot outright, T-Mobile will give them a one-time credit of $300 applied to their bill.

The carrier hopes to eventually combine its network and Sprint's over the course of the next three years, a process that Kuoppamaki says is on track even amid the pandemic.

As for future 5G midband cities, Kuoppamaki wouldn't reveal where the carrier will go next, though he says it "will move fast."

"We're moving as fast as we can, this is what drives us," he said, adding that the carrier's "5G for all" slogan is "not a tagline without substance, we really stand behind that."