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@Momark78 @Joshr1985 . What type of NBN connection do you have?
Have you done a speed test recently? Do the speed test on TPG support page using an ethernet computer. Or 5 GHz wifi device next to router? Still getting proper speed? Rule out problem in external network.
Some routers have better wifi performance than others.
The 2.4 GHz band has longer range than the 5 GHz band. You might be able to swap some devices to the 2.4 band.
Otherwise, a simple wifi extender might suffice. This picks up the wifi signal from the router and extends it. It is placed part-way between router and the low coverage area.