RT-AC66U gives slow broadband connection - FIXED

[sads45]

My ISP has just upgraded my connection from 8Mbps to 16Mbps. If I connect my PC directly to the ISP provided modem I can achieve 16Mbps. If I connect via my RT-AC66U I only achieve 6.9Mbps, which is the same as before the upgrade. I have AsusWRT-Merlin build 380.57 firmware installed. I am fairly new to this so am not sure where to start looking for the cause. Any help appreciated.

 

[TheManFromUncle]

Have you tried using a different cable between the modem and 66U? Try a Cat 6.

 

[sads45]

Have you tried using a different cable between the modem and 66U? Try a Cat 6.

I just tried a different cable but it made no difference

 

[TheManFromUncle]

IF there is such a function as NAT activated in the modem, turn it off. Quite mundane but you've probably already thought of it.

 

[sads45]

I have resolved the issue. I disabled QoS on the RT-AC66U and I now have 16Mbps broadband.

 

[TheManFromUncle]

QoS seems to be the culprit of many problems.