I have checked your NBN-FTTP 50/20 service on our systems and all tests are passing with your modem up time for 1 week with 0 drops.
FTTP is one of the most stable types with minimal faults.
It may be a modem or wifi issue.
If you are using an older NF-12 modem, it may need to be upgraded to a newer model.
Also, try running some speed tests with a device connected directly to the modem via a network/LAN cable.
Wireless interference can cause various issues with your WiFi network. If all of your devices connected to via WiFi are affected, then you may need to change some wireless settings. We've created an article that will guide you on How to improve your home network.