Hi @patrickm.fswvb6 and welcome to the Community!
Republic provisions phones for cellular coverage with one of two cellular network partners. One partner operates a GSM network and is T-Mobile. The other partner operates aCDMA network and is Sprint.
The above said, there’s more to Republic’s blended WiFi/cell service than verifying a particular phone is hardware compatible with one or both of Republic’s cellular network partners. Republic blends the Voice over Internet Protocol (VoIP) network of its former corporate parent Bandwidth.com with one of the two aforementioned cellular network partners.
This blending of networks requires one of the carrier agnostic factory unlocked phones listed here: Republic Wireless Phones – Republic Help. No phone not on the list will work with Republic.