I am trying to build an app and the app is running fine on the emulator ( Sign in and Sign Up are working fine with the API) but when I build the app for release and install it on the device, the API does not work (can not Sign Up and Sign In). Any help on this.
You need to sign your release build. After signing that build, take the SHA-1 and SHA-256 keys used to sign that APK, and add them to your firebase console if you are using firebase.
If you aren't using firebase, you need to post more details regarding your error. It is most likely due to Cleartext HTTP traffic. and you need to add android:usesCleartextTraffic="true" in AndroidManifest.xml.