Thanks for your query!
I checked the NBN portal for your HFC service and tests are passing. It shows the NBN-NTD has been up for 7x days.
You already have a new modem which is great, and an NBN tech visit was recently attended also.
You may need to investigate any localised issue.
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.