FWIW, this is not normal or expected, most users get their notifications more or less instantaneously. Maybe knowing this might help you search for information about whatever it may be that prevents this from working for you.
In that case, is Google Play Services allowed to run in the background / unrestricted / whatever? It is the means to delivering notifications for most apps.
Apply the steps on https://dontkillmyapp.com/ to Google Play Services and Google Play Messaging, that may help. Also check if you have an app running in the background that eats all of your RAM and forces the OS to clear up scheduleable tasks.
Thirty minutes is way longer of a delay than I've ever seen. You may want to try a factory reset in case it's a ROM issue. Fairphone has had its ups and downs over the years when it comes to the built in software.