If you haven't done so already, I'd say it's time to call Quicken Support on the phone and request help. Let them do a screen share session with you to review this Online Biller issue on your computer, to determine the nature of the problem and to figure out how to fix it.
Quicken Support:
For whatever it's worth ... I would not bother attempting to activate any loan or mortgage account for downloading. (Not discussing LOC or HELOC accounts here)
An online-connected loan or mortgage account does NOT have a transaction register. All data shown in the account come from whatever information the bank downloads to you ... if this process works at all.
As a result of being connected, the scheduled payment transaction reminder cannot transfer the amount of principal paid into the (non existent) account register and must use a category, usually something like Loan:Principal, instead.