Welcome to the Community @saidiksha
If you are having problems in connecting to the 5GHz WiFi network, then you might need to check if your device is compatible with the 5GHz network.
Once confirmed that it is compatible, then make sure that you are using the correct WiFi password to connect to it. If you forgot about your WiFi password, we've created an article that will help you retrieve it. Here's the link : Forgotten Your Wi-Fi Password?
We've also created an article that will help you personalise your WiFi password. You may refer to : Changing your WiFi password on your TPG Modem
In case you experienced an issue with your WiFi connection, we've created an article that will guide you on how to improve your home network.
Feel free to search Community for help with your issue, or contact us and we'd be glad to assist you.