I can confirm that Verizon (prepaid in my case) works. I just had to activate the SIM on another, supported phone, then switch it over.
And since I don’t think I’ve seen anyone else mention it here, do note that T-mobile is shutting down their 4G network starting this year, and the shutdown will be complete by 2028. They’re betting on their T-satellite service for serving areas that don’t get 5G. Anyone hoping to use Kompakt with T-mobile should start looking into other options within the next couple years.