@HansWegerzynste 

To configure TD-W8970 as an access point (there's no dedicated AP mode in this device) you'll need to do the following:

Set the TD-W8970 in ' DSL Modem Router Mode '.

Set its LAN IP address to 192.168.0.2 and disable its DHCP server. (These settings are in Network => LAN Settings menu).

Then connect TD-W8970 to your C6 via one of its LAN ports - (Case 2) in this guide.

Try connecting a device to TD-W8970 wireless SSID and check if you have an Internet access.

If you have Internet access skip the next two steps.

If you don't, then instead disabling TD-W8970 DHCP server, select 'DHCP Relay' and set the C6 IP address - 192.168.0.1 in Remote Server Address field.

Again, try connecting a device to TD-W8970 wireless SSID and check if you have an Internet access.

If everything's OK you can continue to configuring WR841N as a range extender to TD-W8970.

Depending on WR841N HW version the available options are:

1. Configure WR841N WDS feature (older HW releases).

2. Configure WR841N in Range Extender mode (newer HW releases).

  @HansWegerzynste 

Yes, that was what you were doing in 'Wireless Router Mode' - this introduces a double NAT in your network (once from C6 and another from TD-W8970).

Set the TD-W8970 in ' DSL Modem Router Mode ' - this way the TD-W8970 WAN port won't gen any IP address which will conflict with its LAN IP address 192.168.0.2.

In 'DSL Modem Router Mode ' you can use any of the TD-W8970 4 LAN ports.

These settings are tedious if you're using a phone to configure - better use a computer connected to the router's LAN port (that includes WR841N too).