i've used a 5m LAN cable to test the broadband speed for my 300mbps broadband package, but i only got 30mbps ~ 40 mbps speed only...

is it a limitation on the router? or time broadband issue?

I thought I'd share this since it's not obvious to a lot of people. In fact, even I didn't realise it until a friend made me aware of this last week after he inquired about downgrading his package.
Note the bolded and underlined part.

He signed on to the RM179 package six months ago, and wanted to downgrade to the RM149 package because the new upgraded speed of 100Mbps for the lower plan was more than enough for his needs.

He expected to have his contract renewed/reset to 24 months, but was shocked when he was told that it was 24 months on top of the 18 months he still had left on his current contract, making it 42 months . I don't think even Unifi does this for service downgrades.

So, there you go, when picking a Time Fibre Broadband package, be sure of the speed you want and can commit to paying for it for the duration of your contract, because a downgrade can and will be very painful.

Needless to say, my friend was and will not hesitate to look for alternatives when the time comes.