You're in the right place. Welcome to Community.
The TalkTalk Support Team will need to verify you as the registered user and then raise a ticket for the email team to change the password for you. This is just because the mailbox is on a legacy platform rather than on the new TalkTalk Mail platform where mailbox management will be under your control.
To get the process started, I'll alert the TalkTalk Community team for you.
In the meantime, please update your Community Profile Personal Information (Click here) to include your name, current contact 'phone numbers and address with postcode (location) then scroll down to Private notes and add the onetel email address and Save changes.
A TalkTalk OCE will reply to this thread first and then Community Personal Message you to verify the registered email user details.