How to add existing Cellular (AT&T) data on iPad Air

Question

Just bought/received a new iPad Air (AT&T).
Trying to get the AT&T data portion working.
I go to Settings, Cellular Data, View Account, Cellular Data Account....
I have 3 choices:
Set UP New Account >
Add to Existing AT&T Account> 
Transfer Service From Another iPad>
Since I already have 3 Iphone 5's on an UNLIMITED DATA plan with AT&T (Grandfathered-in), I chose the middle selection.
It then asks for the following:
Cellular Phone: Required
Zip Code: Required
Passcode: Required if enabled
SSN:  Last four digits of SSN
I entered EVERYTHING correctly, but it comes back after just a few attempts:
We're sorry, the maximum number of authentication attempts has been exceeded.  You must wait 24-hrs before trying again.
NOW.....Is the PASSCODE that they are referring to the AT&T PASSCODE???  Or the silly passcode to open the iPad after the finger swipe?
This is the ONLY thing that I can think of that may have been entered incorrectly.  I was entering in my AT&T PASSCODE (4 digit) security passcode that you use to login online.
IF they are asking for the APPLE PASSCODE that you enter after you finger swipe, then maybe that's the issue???
HELP!!!!

Answer 1

sberman,
When I log in to the link you specified, I have to enter in my:
USER ID:  (Phone number)
and
PASSWORD: (Yes much longer than 4 numbers)
then....
I have a PASSCODE that I must enter (4 numbers) as a further security wall.
So....when setting up this AT&T Data plan on the iPad, it's asking for a PASSCODE (Required if enabled) I am assuming that it's the 4 digit number, and NOT the PASSWORD that is alpha-numeric and much longer.