Here's what I did and it worked perfectly.
Turned off the cellular data on my IPad 1
Went to settings, cellular data on my new IPad
Clicked on Cellular Account Data
Entered my user ID and PW from my AT&T account
Checked to see that the "DON Unlimited MB for 30 days - LTE domestic" was checked.
Clicked next (scroll down to see the button)
Waited 30 seconds and received a message that my account was active.
I then logged out and then logged back in to ensure that the phone number transferred and it did.
No issues, whatsoever, it works fine.
Turned off the cellular data on my IPad 1
Went to settings, cellular data on my new IPad
Clicked on Cellular Account Data
Entered my user ID and PW from my AT&T account
Checked to see that the "DON Unlimited MB for 30 days - LTE domestic" was checked.
Clicked next (scroll down to see the button)
Waited 30 seconds and received a message that my account was active.
I then logged out and then logged back in to ensure that the phone number transferred and it did.
No issues, whatsoever, it works fine.