It would be best if you put your PPoE DSL modem in gateway (or bridge) mode and let your 825 handle the firewall, QoS, DHCP, and other major functions. After all you did BUY the 825 because it's a really good Wireless router, right?
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To connect two routers together, it's as simple as connecting port 1 on the 825 to port 1 on the 2nd router.
Just make sure on your
2nd router:
DHCP is disabled,
The router IP is outside the 825's DHCP Range (e.g. 192.168.0.200 is acceptable)
Any firewall or SPI is disabled
(If it's a wireless router) The SSID & Wireless Security settings are identical but on different channels.
Do not use the WAN portJumbo frames are going to entirely depend on the 2nd router's capabilities. If it's another 655/825/855, Jumbo frames will be automatic.