TO DOs and Notifications
To be able to see all your Trello tasks/cards in To Do you can just navigate to the Reports > TO DOs page.
There you will see a tree for all organizations, boards and cards in To Do.
To get notifications about any task/card in To Do list you can set-up notification period on the General settings page
under the "Notify about any task on To Do lists" section by selecting Every 1 hour or Every 24 hours
You will get notifications in 3T and Slack.