You said " I'm using a TL-WR702N in client mode to provide WLAN access for my PVR without an WLAN interface" I think you should enable DHCP in your WR702N. And assign a fixed DHCP ip from your WR702N...

http://www.dd-wrt.com/site/support/router-database Go to this link and there type WR740 select your version file and download the webrevert .bin file(Download the correct version of firmware...

Ask it from Technical Support.

Try Universal Repeater.

Reset your router and then only change the router setting under Network Tab,Wireless Tab and DHCP Tab if you want and don't touch the other settings.I am only to change major settings.Don't touch...

And another thing you need to know that Archer C7 dosen't have ADSL port.So if your ISP in giving you ADSL connection so you need something that converts your ADSL connection to RJ45 port