Please be aware that the layout and steps may vary for different Android devices. It is advisable to consult the manufacturer's page for information specific to your device.
Open up your settings, and search for "mobile network".
Press on the Mobile network.
Press on Advanced
Press Access Pont Names (APN)
Press on the APN you are currently using.
Enter IP, Port, Username and password into correlating areas
Make sure to save changes.
Turn off and turn on mobile data (wait 5-10secs)
Open up your browser and enter the username and password of the proxy again. See the picture above.