Probably it is due to the distance from your house to the exchange... You can check whether your line is 50% fiber optic (from the exchange to the remote cabinet DSLAM) or purely 100% copper by accessing TM SAMS WEBSITE. Just enter your streamyx username or phone number and you will know it. For partially fiber line, the green F symbol will appear at the J or Jumper area, meanwhile for the fully copper line, the green TICK symbol will appear instead of F symbol. You can see the attachment below for the example when your line is partially (or 50%) fiber.

It is recommended that you change the modem because you said that your house is big. You're using 4Mbps, and I heard that there are many problems involve around the TM Wifi modem (especially Riger and Innacom one). But you need to spend a bit... Maybe TP-Link TD-W8960N is suitable for you....    Depending on the seller, and the most cheap one that I found is RM122 from one reputable seller at Lelong.my... Or perhaps, you can do the survey at the Lowyat itself...  

[attachmentid=2560733]