Samsung
Change Default Messaging App auf Samsung
On Samsung phones, incoming text messages open in whichever app is set as the default SMS handler. If a third-party messaging app is installed but never receives texts, Samsung is likely still pointing to its own Messages app.
Change this in the system app settings, not inside the messaging app itself. Open Settings ➔ Apps ➔ Choose default apps ➔ SMS app, then pick the app you want to handle text messages and confirm the change.
This setting only affects standard SMS and MMS. It has no effect on messaging apps that run entirely over the internet, since those never register as an SMS handler in the first place.
One side effect to know: if your Samsung device has a screen lock and gets restarted, no messaging app - including a newly set default - loads notifications until you unlock the device once. This is a security measure and has nothing to do with which app you chose as default.
If switching the default app doesn't fix missing notifications, check that the new app has notification permissions enabled separately. Android treats default-app status and notification permissions as two independent settings.