To be able to get notifications when using the mobile app, you need to have the Task Reminders strategy turned on. You can do that by opening the sidebar when using the mobile app, tapping on Account/Settings, and then on Strategies. It's also possible to activate it on desktop/web app by going to ☰ 🡒 Strategies 🡒 Task Reminders.
Once you have the strategy turned on, in order for reminders to be successfully created, they need to satisfy these two conditions:
Be scheduled for today or in the future
Have time in their title
To see the list of all reminders, open the sidebar, tap on Account/Settings, and then on Reminders.
Reminders aren't getting created even though they have time and are scheduled?
If this is the case, make sure that "Create automatically" option is turned on. You can do that only on the web/desktop app by going to ☰ 🡒 Strategies 🡒 Task Reminders and clicking on the settings icon: