iPhone 12 purchased in Taiwan and use it in the US

I'm currently using iPhone 7 and my phone has gotten to the point where I need to get a new one (i.e. can't pick up calls even if I restart it)

Given I'll be returning to the states in August I'm considering an iPhone 12, yet the Apple staff in TW advised me to not purchase it now as the models are different, and he was not sure if the TW version would support the broadband in the states. To be honest I have no idea what the concerns would be if I buy the sim-free version. I also don't consider switching to 5G anytime soon in the US, just wanna know if a sim-free iPhone 12 Taiwan version will work fine in the US? As in supporting LTE etc.

Would love to know more about the pros and cons of getting an iPhone 12 outside of the US as I'm really not tech-savvy!