I have a Samsung Galaxy S7 and I use the S-Planner app. I have my contacts NOT on Google (and I don't want to share them with Google). My contacts are saved as "Samsung account" contacts.
I have contacts with a birthday, and the birthdays are correctly shown in the S Planner app. I also have notifications for all-day events on in the S Planner app. Still, I'm not reminded for birthdays.
How to get reminders for birthdays like for all other calender entries?
There are similar discussions for the Google calender here.
Even there is not a real solution named just workarounds.
However there is recently also an app Contact Events & Birthdays referenced, which I found quite usefull and solved this problem for me. Basically the reminders are then created by this app (and upcoming events can be listed). The app is designed to scan the adressbook for Birthdays and other events.
As you saved your contacts as "Samsung Contacts" I'm not sure if it will work for you, but maybe it is worth a try.
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