iphone xs max

If it's really unlocked, you should be able to use SIMs supported by Apple. Check this list in the country you're in: Wireless carrier support and features for iPhone in Europe - Apple Support

or, if you're trying to use an eSIM, this one:--> Find wireless carriers that offer eSIM service - Apple Support

Regards

Giulio

Best buy does not sell unlocked devices. Devices purchased from best buy can only initially be activated on a US carrier, and if eligible, can then be unlocked.