Mobile network won’t allow me to enter so cell phone carrier turn on.

I bought the Cat S53 off eBay from a person in Europe I’m from the US the phone is in good sharpe New”.. So I took the phone to AT&T which the phone supports the carrier. AT&T put the SIM card in the phone works it make and receive calls but can’t access the internet. When they went into Network & Internet, Mobile network won’t allow me to go no further. I master reset did all the reset and still can’t access Mobile network. My name is Levi Billups can be reached at 443-415-3305