Microsoft shut down mobile push notifications for Windows Phone 7.5 and Windows Phone 8.0 on February 20, 2018 for all apps.
This is what happens, in case you have an old WP 7.5 or 8.0 device:
It's highly recommended to upgrade the OS to at least WP 8.1 or even better, switch the OS (iOS, Android...).
Microsoft's official statement: "This wasn't a technical decision, it was a business decision." Link