I just got an AT&T SIM and was looking for an app to manage the account.
A search came up with myAT&T but when I try to search the App Store on the phone I get dozens of irrelevant apps, but NOT what I was looking for.
How can I find EXACT app in App Store
EDIT
I searched in Safari (on the phone with the AT&T SIM), and clicked the link in the answer, but the result says "This app is only available on the App Store for iOS devices"
I am in the US - connected to AT&T, running iOS 10.3.3 (the latest for the iPhone5C)
I can launch the App Store app, but there is no download link, and it states "This app is only available on the App Store for iOS devices".
On my iPad I get an error "The item you've requested is not currently available in the Australian Store" - so I guess my problem then becomes how to use the US Store.