Only legacy models qualify for the outstanding Republic Refund plan and these phones only work with Sprint. The best of these are the Moto X (1st Gen), Moto X (2nd Gen) and Moto G (3rd Gen). These models are usually available on eBay and Swappa.
The Phones you see in the Phones section and the current plans in the Plans section are what is available now. These phones work with T-Mobile. Some of them work with both GSM (T-Mobile) and CDMA (Sprint) but at this time new customers are sent the GSM configuration.
They do not and most likely will not work like Fi. The phone will be configured for GSM only (now) and GSM OR CDMA at some time in the near future.
Going forward (hopefully in just a few days) you will study both maps, select a phone compatible with the best coverage, select the Phone that will work with that coverage at a price you like, and order the phone.
Here are links to the coverage maps:
4G LTE Coverage Map | Check Your 4G LTE Cell Phone Coverage | T-Mobile