Hi @josephk.gbpxsp and welcome to the Community!
Sometimes there are hardware differences among variants of what would otherwise be the same model phone. For example, Samsung often uses its own Exynos processors in Galaxy S series phones intended for use outside the U.S. while using Qualcomm Snapdragon processors for Galaxy S series phones intended for use in the U.S. Additionally, phones intended for use outside the U.S. (a/k/a international variants) will indeed sometimes lack one or more cellular radio frequencies (a/k/a bands) used in the U.S. The reverse is also true.
The above said, the issue with Republic compatibility generally is a matter of the phone’s firmware. Unlike most mobile virtual network operators (MVNOs), Republic doesn’t merely rely on the cellular network infrastructure of its network partners. Republic blends the Voice over Internet Protocol (VoIP) network of its former corporate parent Bandwidth.com with one of two cellular network partners. VoIP is the technology powering the WiFi portion of Republic’s blended service.
This blending of disparate networks (one VoIP, one cellular) is managed by Republic’s required mobile app. Republic’s app, in turn, relies on certain application programming interfaces (APIs) being present in a supported phone’s firmware. Generally, firmware on phones intended for use with a specific carrier(s) (a/k/a branded phones) lack the APIs needed by Republic’s app.
This is the reason behind Republic supporting only carrier agnostic U.S. factory unlocked variants. While phones that didn’t leave the factory in an unlocked state may indeed be unlocked by the carrier(s) it was branded for, that is not the same as a factory unlocked phone. An after-the-fact carrier unlock does not in and of itself change the firmware on a phone.
In theory, if one is able to access it and has the skills required to do so, one might manually flash the U.S. (not international) factory unlocked firmware to the phone. Manually flashing a phone has risks. If one doesn’t pay attention to detail, one might end up bricking the phone. In any event, while one has every right to assume that risk, one should not expect Republic to assist if something goes wrong. Nor, should one expect Republic to assist in making a manually flashed phone work on its blended network if for some reason it doesn’t.
For what it’s worth, there’s some excellent advice for sourcing carrier agnostic U.S. factory unlocked Republic compatible phones here in the Community: How to Find a Compatible Unlocked Phone.