Thank you for the clarification and no apology is needed!
There are varying flavors of unlocked. There’s carrier unlocked, which simply means a phone originally manufactured for a particular cellular network is now able to be used on other compatible networks. For example, an AT&T branded phone once unlocked might work on T-Mobile’s network. Unlocking a carrier branded phone doesn’t change the software on said phone.
There’s also factory unlocked, which is what Republic’s app based blended WiFi/cell service requires. Factory unlocked phones are carrier agnostic and have no traces of carrier branded software on the phone.
I wouldn’t go as far as to say finding a compatible used S6 is almost impossible but no question there’s a degree of difficulty involved. The key would be to ask any seller to verify the specific software build on the S6 you’d be considering. It must match one of the following:
Alternatively, if the seller still has the original box, ask them to verify the manufacturer’s SKU is SM-G920TZKAXAR. The part after SM-G920T is what distinguishes the North American factory unlocked variant that works with Republic from the T-Mobile carrier branded variant that doesn’t. Samsung made this more difficult than need be by using the same model number (SM-G920T) for both flavors of phone.
The correct S6 is actually still available for purchase directly from Samsung here: https://www.samsung.com/us/mobile/phones/galaxy-s/samsung-galaxy-s6-32gb-unlocked-black-sapphire-sm-g920tzkaxar. I understand the price tag may not be particularly attractive.