SM-G950U (without the 1) is the U.S. carrier (AT&T, Sprint, T-Mobile, Verizon) locked variant of Samsung’s Galaxy S8. It may be carrier unlocked, however, that won’t be sufficient to get it working with Republic. Short of flashing the U.S. factory unlocked firmware (doable but not easy), there is no way to make that phone compatible with Republic.
You do indeed need to source model SM-G9650U1) as that’s the factory unlocked model compatible with Republic. Factory unlocked and carrier unlocked are not the same thing. This article from @louisdi is, in my opinion, the best guidance for sourcing Republic compatible phones: How to Find a Compatible Unlocked Phone.