iPhone 11 Sim Unsupported in SA from USA

Carrier Lock:- Request your iPhone’s original Carrier to unlock your iPhone and if they agree and do it, then you may know the further process here --> How to unlock your iPhone for use with a different carrier ...

Device Unlocking Policies | Verizon Privacy Policy

Unlock a Phone or Device - AT&T Device Unlock Portal

Unlocking your Sprint device - T-Mobile

Unlock Your Phone for Spectrum Mobile

SIM Unlock Policy | Unlock Your Mobile Wireless Device

Where did you buy the device from in the USA?

BestBuy does not sell "International unlocked" iPhones

They sell Universal iPhones (Which can be used with any International carrier after the same is unlocked by the carrier [Verizon, T-Mobile, Sprint, AT&T] on which you activate the iPhone)

Cell Phones Bought for International Use at Best B... - Page 20 ...