I can say it is router issue. Perhaps the model has weak transmit power or/and poor reception sensitivity.

Repeater or booster will just lower your speed more. Reason is WiFi is half duplex transmission. During transmit it cannot listen and during receive it cannot transmit, like walkie talkie.

Imagine from router to your laptop takes 1s and so does from your laptop to router. So, complete transmission takes 2s. If you use repeater/range extender, then now it is from router to extender and then only to laptop. Now round trip will become 4s total.

I know you will ask why I need to buy my own router. The problem is, Maxis or even Tm will have same standard router and the model they used probably get good deal from manufacturer (probably need to clear stock )

I bought my own and has no problem to cover 2 storey house and I can still get about 200Mbps upstairs. My routers is downstairs and located even in front end of the house.