TCP Sockets for React Native App as a client and external server

I need some help in setting up an external server with TCP sockets and a client for my React Native app. I initalized the app with react-native init. This app needs to get messages from a server whenever something in the database of the server changes. After that I want to make a push notification.

I have two options:

1.) PHP without any extra modules I have to activate in a php.ini file. I can't modify it. Only my service provider can (Strato)

or

2.) Any code that runs on Heroku

Any help is welcome. Thank you for your time and help

EDIT: I need the same feature like WhatsApp uses. If someone sends you a message, a push notification will appear.