I have ran into the service menu, I have found the same URL for FOTA but it's quite different.
Here is mine:
with this one, I get redirected to https://xdme.wireless.att.com/jsp/login/login.jsp
and it is asking for username and password, I am unable to provide the username but these are the details in the Service menu:
User Password: 74vvfk5910ol8r8f
Server Password :64vvfk4550ol8r3f
User Password: eimc3cbf83rlri19
Server Password: cimce3cc5a23rlri4d
Client Auth Type: DIGEST
It has two accounts for FOTA auth, then it provides a Client Name which is my phone's IMEI. I tried with that but no luck until now.
Update should be in any location in this server