Hi @tquan,
For customers experiencing any issues with their NBN service, we recommend the best thing to do first is check if there's an outage on the NBN network.
We have created this article that can guide you on How to Check for NBN Network Outage
If there are no known outages in your area, customers can proceed with the troubleshooting steps indicated on this link :https://www.tpg.com.au/support/nbn-hfc-cable/problems_speed_nbn_hfc
We've also created an article that will help you troubleshoot a Slow speed issue. For WiFi connection issues, here are some articles that will help you improve your home network. (Easy Steps to Improve Wi-Fi Connection In Your Home, Wireless Channel on your modem - what is it and why should I change it?)
Feel free to search Community for help with your issue, or contact us and we'd be glad to assist you.
BasilDV
