5G is just marketing, don't worry about it. Your phone will still get faster speeds than a 4G plan on the 4G network.
As @dust2dust said, first check to make sure your phone/account is setup for VoLTE on public mobile. Without this you will still get data and text services in the US, but phone calls may not work. The easiest way to test this at home is to turn off wifi, make sure you are connected to the 4G network, then make a phone call; does your phone stay connected to the 4G network while the call is connected? If yes, you have VoLTE and shouldn't have any problems in the US. If your phone drops to 3G when on a call, you might have issues. I suspect you should be fine though.
