just got my pure300 installed today. nice helpful chap tried his best to help me but nv solved the problem. He so in a rush.
Original setup
ONT>Grey Cat5e>RT-N15U>Blue Cat5e>PC
Problem
MR Test1: Tested via my current old Asus RT-N15U router.
ONT>New Cat5e>RT-AC56U>MR laptop
DL speed of 100Mbps and UL speed of 100Mbps

From my own setup:
MR Verdict: Grey Cat5e cable connected to ONT is faulty.
Not convinced I ask him to test the speed by using a new cable MR provide. As the N15U is in my room and I need to move it to the living room cos the new cable is only 1m long.
However the N15U take forever to reboot which he did for the first test. So he took out a new RT-AC56U router at the living room. Booted up in seconds and with the new cable and Router
MR Test 2: New cable and RT-AC56U Router
ONT>New Cat5e>RT-AC56U>MR laptop
The DL= 300Mbps and UL=>100Mbps.
MR Verdict: Grey Cat5e cable connected to ONT is faulty.
[just thought of this]An issue here that by using a new cable and a new router, what make the test work? the cable or the router?
New Setup
My own Test:
ONT>New Cat5e>RT-N15U>Blue Cat5e>PC
[I run my Blue CAt5e cable out the window through the main door to my router with the new short Cat5e] 
can I be so heng both my Blue and Grey Cat5e cable is faulty?
assuming if the cable has no problems then the only difference between MR Test2 and my orignal setup is the router
HELP!
From what I see, seem more likely that the problem come from the router rather than the cable.
The cable run through my living room to the far corner of my room. Not fun trying to replace 2 cables.
From my description I hope someone can point me to the right direction of the problem at hand. TIA