I got set up with TPG NBN 50 yesterday around 1pm getting speeds of 38 Mbps, from 5pm onwards I couldn't even watch a youtube video with 1 device connected via Wifi, speedtests results 3-4 Mbps from 5pm until 11pm, tested every hour, had to use my phone for a hotspot.
I emailed TPG three times and they completely ignore my email and send me the same automated response every time, very unhappy.
Welcome to TPG Community!
We were able to locate your account using your community details and tested the line.
We were not able to detect any fault within the NBN FTTN network and it appears that the modem is getting a good sync speed.
We've created some articles that will guide you on how to troubleshoot a slow speed issue and improve your WiFi network at home. Here are the links: Easy Steps to Improve Wi-Fi Connection In Your Home , Slow speed issue.
Also, you may check the sync speed of your modem by following the steps here: How to check the connection speed on your NBN FTTN/FTTB service
Let us know how it goes.