Do you mean it is not showing them when the keyboard gets displayed … which it will indeed not do … or do you mean it is not showing them when you long-tap the corresponding base letter (e.g. long-tap a for ä etc.)… which is the normal way to access them?
Please also check the settings there … Einstellungen - System - Sprachen und Eingabe - Bildschirmtastatur - Android-Tastatur (AOSP) - Sprachen