T-Mobile new account number port question

I'm trying to sign up for T-Mobile prepaid service for two lines, porting our numbers in from AT&T, but running into an issue. The option to "use your existing number" isn't selectable during checkout (see screenshot).

I've confirmed the numbers are eligible to port, tried again in an incognito window...no luck.Am I missing something here? Do I select a new number and port our numbers in after we receive the SIM cards and activate? The TM FAQ states to keep our numbers I should choose that option during checkout, so...hoping some of you existing T-Mobile users have some insight.