I recently bought a second 810L to extend coverage with the 5GHz band. I flashed several DD-WRT builds on both routers but had no joy until...I flashed both routers with the updated firmware from D-Link (1.03). I left the D-Link stock firmware on the AP and flashed the repeater bridge-to-be with the firmware on the DD-WRT router database (25815). (
http://www.dd-wrt.com/site/support/router-database).
AP wireless settings for the 2.4GHz band are Mode: Mixed 802.11n, 802.11g and 802.11b; Enable Auto Scan: Unchecked; Wireless Channel: 11; Transmission Rate: Best (automatic); and Channel Width: 20MHz. For the 5GHz band they are Mode: 802.11n only, Enable Auto Scan: Unchecked, Wireless Channel: 5.180GHz - CH 36, and Channel Width: Auto 20/40 MHz.
I then configured the DD-WRT router as specified here (
http://www.dd-wrt.com/wiki/index.php/Repeater_Bridge ) employing the Broadcom instructions. Repeater Bridge wireless settings for the 2.4GHz band are Wireless Mode: Repeater Bridge, Wireless Network Mode: Mixed, Channel Width: 20 MHz, Network Configuration: Bridged. For the 5GHz band they are Wireless Mode: Repeater Bridge, Wireless Network Mode: N-Only (5 GHz), Channel Width: Wide HT40 (20+20 MHz), Extension Channel: lower, Network Configuration: Bridged.
Joy!
Hope this helps anyone considering extending their network.