Will a CDMA carrier's LTE work on a GSM-only phone? More specifically, I'd like to know if a Sprint-based carrier (Tello) will work on a Xiaomi Redmi Note 8 (M1908C3JG).
No

Will my phone work says Redmi Note 7 does not work on Tello.

Readme Note 7 has more operating frequencies than Readme Note 8 (see Network section- 
Note 8 frequencies are a sub set of Note 7 ) but Readme Note 7 doesn't work with Tello.

If you switch carriers, you have a 25 % coverage as indicated here, but that would need you to do homework to find out which carrier and if that frequency (s) are used where you operate