Please allow me to try again. Republic offers multiple coverage options for your husband’s Samsung Galaxy S8. Republic provisions phones for coverage with one of two network partners. One partner operates a GSM network and is T-Mobile. The other partner operates a CDMA network and is Sprint (same as Boost).
Presuming you purchased Republic’s standalone SIM for use in your phones, it provides GSM (T-Mobile) coverage. It’s possible to switch your husband’s S8 to CDMA (Sprint) but doing so requires a different SIM as outlined in the Help Center article I referenced in an earlier reply. For convenience, I’ll do so again here: How to Request a 3.0 CDMA SIM Card – Republic Help. A Republic CDMA SIM would give your husband the very same coverage he had with Boost.
Were you using the OnePlus 6T with Boost? I ask because, to the best of my knowledge, OnePlus phones are GSM network only and, therefore, cannot work with Boost or other CDMA service providers.