@rosfek I am guessing that for some reason Optus have not linked the two accounts, perhaps when data has been entered an error etc.
You are going to have to contact Optus, you can message them from the My Optus App, Facebook or Twitter.
If you choose the phone option I have read reports it can be a long time on hold and you are best to ignore the initial prompts to dial 1. Also just in case you unaware if the bill has already been issued and a direct debit has been setup you will need pay that bill manually and then the direct debit works for subsequent bills. It is a quirk of the Optus system.