How to mark all unread notifications as read

Hello! I would like my badge in the upper toolbar to stop showing 99+ and show 0 instead, but I've got 2756 unread notifications and I can't figure out how to mark them all as read. I've gone into some of the groups I subscribed to and found the "Mark as read" button under "More", but it never changed the results (still 2756). I have now turned off all notifications, including email. I figure there's gotta be a way to mark them all as read.

Thanks for any help!