Someone at HBO doesn't know what they are talking about.
I just checked the iOS App Store. All iOS HBO apps still support iOS 8 and later.
Are you sure your subscription/cable bill is paid and up to date as these apps need a paid cable and/or iOS monthly subscription?
Have you tried a hard reset of your iPad by holding down both the Home and sleep/wake buttons simultaneously until your iPad goes to black and restarts with Apple logo, then release the buttons?
Have you tried deleting the app and reinstalling it from either the iOS App Store or from the Purchases tab in the lower menu in the iOS App Store app?
Your HBO GO app should still be working if everything has been paid up to use this app.
Best of Luck to You!