I took your advice and enabled the QoS but it didnt not help at all. Let me re-explain my issue.

my connection is now 50mbps down. back when I used to have a 30mbps download speed, My download was certainly slower but it was fine. the internet was usable and everything was ok.
When I upgraded my connection, a 1mb download speed not even higher, will render my whole connection unusable for all the computers except for the one downloading, and the one downloading cannot even browse. only the download will keep going. I cannot access the router via 192.xxx.x.x nor access any other website. With or without QoS configured.
It just seems like the whole router cannot handle the load. I cannot view the logs during the download coz i cant access the router during. So how can I investigate further?