Hello Piopilpulcio,
The first thing we would suggest would be to power cycle the iPhone by following these steps found at Restart your iPhone - Apple Support:
1. Press and hold either volume button and the side button until the power-off slider appears.
2. Drag the slider, then wait 30 seconds for your device to turn off. If your device is frozen or unresponsive, Force restart iPhone - Apple Support
3. To turn your device back on, press and hold the side button (on the right side of your iPhone) until you see the Apple logo.
While simple, this can refresh the iOS, apps and connections while resolving some issues.
If the issue continues, confirm that notifications are enabled based on the information found here: Change notification settings on iPhone - Apple Support
Once you have confirmed everything is correct, if the problem is isolated to the single app, the next step would be to reach out to the app developer: How to contact an app developer - Apple Support
Cheers