@Solla-topee 
1. Archer C6 is in access point mode, the DHCP server is on the main modem/router (I left it on auto on the C6 because I'm sure it's not assigning any IPs, the client list is empty).
I set the same SSID and password on both.

2. Realtek PCIe GbE Family Controller. Tried with another computer, still 1Gbps connection, same result. The speed fluctuates greatly, from 4Mbps to 150Mbps in seconds. It peaks briefly at 150-160 Mbps, so the connection is not strictly capped at 100Mbps as i previously thought.

3. Here are the images. 

@Solla-topee 

hi

I am facing the same issue where speed through the router is getting capped at 10Mbps. I have a new archer C6 router. It was working fine and suddenly I started facing this issue.
Please let me know if there's a solution to this issue.

Thanks

@Solla-topee I've been using the solution provided by your engineer for the past few months. His solution was to set manually set the parameters needed for the router to act as an AP while still being in router mode. This solution, while it solves my LAN speed issue, doesn't use the WAN port on the router, so the router itself is not connected to the internet. This creates numerous new issues: limited use of the Tether app (cannot login as Cloud Device while not at home), no online upgrade, no automatic time retrieval. This last thing is quite important to me because the router relies on current time to turn LEDs and WiFi on/off automatically, and without automatic sync I have to set it manually frequently cause often it becomes completely wrong (e.g. after reboot or power loss, or sometimes even without an apparent reason). By the way, why is it not possible to turn LEDs and WiFi on/off automatically while the router is set in AP mode? The option is not there.
To summarize, what I'd like to know if it's possible to:
1. restore full LAN speed while operating as AP (I get steady 30MB/s speeds in router mode, whilst in AP mode LAN speeds sometimes are under 1MB/s)
2. add the option to turn LEDs and WiFi on/off automatically while the router is set to AP mode

@Kevin_Z By manually changing the C6 into an access point (ethernet from main router connected to C6 ethernet port, dhcp off, static ip in same subnet of main router) you can actually use LED control and wireless schedule. These features are not available if the C6 is actually set as AP the "intended way", in the operation mode menu. Is there a reason why these features are disabled in the proper AP mode? And why are LAN transfers so slow in AP mode?

@Kevin_Z any solutions?