My sister's home in Seremban is using Streamyx 4Mbps. The residence area which is 1km away has UniFi ports but not her area. Anyway, her modem had issue and she just changed to a modem+router. Now the problem is it keeps disconnecting... Like every half an hour once or something.

What could be the cause? Earlier didn't have this problem, only with this new router. It is a TP Link TD-W8901N model. Already updated to the latest firmware and reset for gazillion times already.

Which area? Town or kampung?

Well, I just upgraded from 4mbps Screamyx to unifi VIP5. Download speed do improve slightly, from 410kb/s to now stable at 500-550kb/s.
Used on 10 and 20mbps package in Qatar before, anything > 10mbps you won't really see improvements as you will be limited by servers, like downloading from some file sharing sites, the most they can go is 900-1000kb/s.