The weekly notification about restarting the phone is now missing on my Note 9. Unfortunately trying the solution from the Galaxy S9 doesn't resolve it because there are no settings for Diagnostic Notifications. If anyone knows, please let me know!
Are you asking about a notification or the setting to automatically reboot your phone??
To set up the automatic restart, Settings > Device Care > three dots upper right corner > Advanced > Turn on Auto restart and set your date and time you want the device to restart.
If you use your phone as an alarm clock or for notifications, the you will need to go to Settings > Biometrics and Security > Secure Startup and turn this feature OFF. This feature locks your phone down until you manually re-enter your passcode. The quote from this page says "You will not receive any calls, messages, or notificaitons, and alarms will not sound, until the phone has started back up."
I'm asking about the notification reminder for it, I prefer doing it on my own terms.
Never seen the restart reminder unless you can set it up through the Alarm app or the reminder app or create a reminder though the Calendar program. (I like Googles Calendar. Got a couple of dozen different reminders set up through there.) I like the auto restart. Hits every sunday morning at 03:00 hours.