Yes Note 20 5G and Ultra are 5G Capable phones but depends on your service plan, best bet is to contact your carrier, in your case AT&T
No. All Galaxy Note20's have all 2020 5G bands built-in (even mmwave). But you will need to flash that carrier's firmware to get full use out of it. You can use AT&T's network with the phone sim unlocked but you will not get access to a few account features (like HD or wifi calling) or perks without the firmware and their AT&T apps running on it. C-Band is considered a 2021 band and was not considered in the modem design.