Experienced this problem with my D6400/AC1600, after changing from ADSL2 to FTTC.
Working fine on ADSL2, with latest Firmware Version: 1.0.0.56_1.0.56, I have just moved onto FTTC with a 40Mbps Download/10 Mbps Upload profile.
All speed testing returned a nice approx. 37Mbps Download but a frustrating just under 1Mbps Upload!
Despite this the 'Uplink bandwidth: Maximum Speed Test' on the 'QoS Setup' page would show a sync. between around 9 to 10.5Mbps.
The ISP ran tests and I performed the usual swap-outs of Microfilter and connection cable into the Master Socket with Face Plate removed, turning off all WiFi and working from just one LAN wired Desktop. All this and numerous re-boots was to no avail with no improvement in the Upload speed and a pending temporary test router on the way.
I ran ping tests in Terminal to establish the maximum MTU without fragmentation, getting 1464 this was entered into the router, producing no change in speed test results but a now VERY consistant 10.75Mbps in the 'Uplink bandwidth: Maximum Speed Test' on the 'QoS Setup' page.
Then I read some forums with people experiencing similar with other makes of router, with "QoS" being mentioned.
I turned off 'Internet Access QoS', the router rebooted and success, speed tests returned Upload figures of over 9Mbps.
Then I turned off 'WMM (Wi-Fi multimedia) settings', turned back on 'Internet Access QoS', the router rebooted and again over 9Mbps was being returned in speed tests.
Finally I turned back on 'WMM (WiFi multimedia) settings' and after yet another reboot, with both QoS features now turned back-on speed tests are giving consistant Upload figures aroud 9.5Mbps.
I hope it works for any of you with the same problem and Netgear can prevent the problem in a Firmware Update.
