st3fx beat me to posting this by a few minutes-- in the interest of not having 2 threads, I'm going to delete the one I started, and copy my original comment here:
TLDR: Sprint CDMA-only devices are expected to continue working on Tello through at least mid-2021, and Tello will provide at least 3 months notification before ending legacy Sprint service..
Sometime in the not-too-distant future, T-Mobile service will be an option on Tello. 5G access will be included.
and
"Tello’s current plan pricing and options will remain unchanged through the transition process. " "
Looks as if the Moto E 2nd gen phone my very elderly mother-in-law has on Tello will continue to work on Tello for some time, unless the battery gives way. I guess you could say, getting a new $30 phone with 4G LTE + VoLTE that is still going strong several years later, is an enduring benefit from the R+ days.
My thinking is that since all CDMA phones have GSM radios, it is possible to use a GSM SIMcard in a CDMA phone. Long ago, there was a SlickDeals thread about using a FreedomPop GSM SIMcard in the Moto E 2nd gen phone that you mention above.
Not sure, but from what Tello said, it seems the key to transitioning the a Moto E 2 to Timo is VoLTE support, which is necessary along with having the necessary GSM bands. The 4 G LTE version of the Moto E 2 was a bit ahead of its time for a budget smartphone it seems. My concern is how long the battery will last and how easy is it to replace.