Reminder is a feature for the pro version, please upgrade to find this feature.

Web/Mac/Windows Version

Move your mouse on the todo and click the ···button on the right.
Then tick the Remind me and set remind time, choose repeat duration.
At last, never forget to save all the settings.

Android/iOS Version

Click the todo description to enter the todo editing view.
Enable the Reminder, then set remind time and repeat duration​.