I installed a firmware image called Archer C7_V2_150427
and now it seems the problem is gone. After browsing through the download area I found that the build number seems to correspond to the compile date of that file.
And the old one - sic - had the build number 131217 and if I am not mistaken this is from 2013, probably not even RTM firmware.

I don't know who to blame, it was an Amazon best price offer (and there are some reasons why the stuff is cheaper like they have 2 year old firmware)

Currently the speed on 2.4 GHz is now the same doesn't matter if the WDS client is connected or not.

HOWEVER the new firmware isn't without bugs - the autodetection for the uplink failed. After removing and plugging the ethernet cable several times it finally detected that the connected device is a switch... older devices require a cross link cable for that but the AC1750 has autodetect for that.

Performance is now the same, 30 MBit on 2.4 GHz regardless if the other router is active or not.
But I have to mention that Intel Centrino based Wifi devices sometimes do NOT detect that the AC1750 has Mimo and then the performance drops to 30 Mbit. This is visible when it is not connected to the "emulated" AP name but to something called "_2"

After reconnecting several times I am back at 85 Mbit on 5 GHz which is the max of my internet connection, and it isn't faster when I connect my laptop directly to my cable modem.

Newest firmware is: [TABLE] [TR] [TD="class: Item"]Firmware Version:	3.14.3 Build 151014 Rel.49676n
Hardware Version:	Archer C7 v2 00000000

[/TD]
[/TD]
[/TR]

[TD="class: Item"]
[TD="colspan: 2"][/TD]

[/TABLE]

thanks for the information, but this firmware is not listed in the Archer C7 Hardware V2 list.
At least it is not here:

http://www.tp-link.de/download/Archer-C7_V2.html#Firmware

However the file you mentioned seems to be available for US customers only... I don't know what's the difference but the firmware file has a suffix _US

http://www.tp-link.us/download/Archer-C7_V2.html#Firmware

Is there something special inside or something missing?


And the V3 users (using the german website) don't get anything for the V3 hardware version... maybe a bug?

I can´t help you to find a solution for this bug, but I can confirm that issue. I think it is not only a WDS problem. In my house (I am from Germany) I use the TP Link VR900v as Router and to connect to the Internet. As WDS Repeater I would use the Archer C7 - but the datarates seems to be very low after the devices are connected.

The setting is the following:
VR900v: IPv4 192.168.2.1 Subnetz: 255.255.255.0. It is also the gateway and the DHCP Server is of course on. WLAN Channel are 4 (in 2,4GHz Area) and 36 (in 5GHz Area). Because of WDS I can´t use the Auto Channel mode - that is a very big restriction instead of the AVM FritzBox model (the Base can use Channel 6 for example to connect to Clients, the Connection between Base and WDS Repeater can use Channel 10 for example and Clients at the WDS Repeater can also use Channel 1 for example. I think AVM called this function "Cross-Repeating"). Of course is AP isolation turned off.

Archer C7: IPv4 192.168.2.2 (LAN Setting), also the same subnet, DHCP is of course off. WDS information are still entered here because I read that only the Repeater needs the information to work as WDS Repeater. The SSID of both devices should be the same, also the Key. But I also tried a different name with the same result.

The problem is that if the C7 is on, all the devices slow down to 5,5 MBit on WLAN connection - also if they are connected directly to the VR900v. Also a lot of the devices can´t get a correct IP configuration over DHCP, only the manual function worked. It does matter if the are connected to the C7 or the VR900v. Only if they are connected to the C7, they have this issue. The problem seems to be that they won´t find the real DHCP Server on reasons I can´t understand.

But thats not only a WDS problem. Even if I use a patchwork cable to connect both routers and plug the cable to the WAN port of the C7, I have the same problem. Of course the LAN IP is in the same network, also the DHCP Server on the C7 is turned off.

I also have the hw rev 2.0 eu model ;)

Okay I found the issue. WDS is activated on 2.4 GHz and 5 GHz to the base station. And that is the problem, because some pakets will be dropped. If I only activate WDS on 2.4 GHz, it works.

Tried this with two identical AC1750,but that didn't work (and doesn't appear in the WDS specification for 802.11 ac as well) , tried with firmware from dec 2013.
Will try that with newer firmware again... but I don't need the combined bandwidth of both channels. Till today it is "nice to have" but not mandantory

So I have WDS bridge on 5 GHz only with 300 Mbit effective bandwidth... enoguh to bridge an 120 MBit cable connection... which is fair enoguh becasue there are two 1 ft thick concrete walls and 30 ft of distance between the accesspoint and the bridge.

if I upgrade my cable connection to 400 Mbit I would need to have dualchannel :-) and when this doesn't work then I'll get two of the AC2600 series.