networking – tplink router: how to route default traffic to another specific IP

I have a tp-link router TD-W8960N 300Mbps Wireless N ADSL2+ Modem Router, which I use to connect to the internet via PPPoE. I have a backup internet connection with its own router, connected to the tp-link one through a LAN cable.

backup router

What I want is changing the routing table when the first PPPoE connection is down, and routing all the incoming traffic on the tp-link to the second router, but I’m not figuring it out how. What I did is connecting via the telnet and do the following:

 > route show
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface     *      UH    0      0        0 ppp2     *        U     0      0        0 br0
default         *              U     0      0        0 ppp2
 > route delete default
Invalid destination IP address
 > route add default
Invalid destination IP address
 > route add
Please at least enter gateway IP or interface

I also tried to add it via the HTTP interface, but it tells me that is an invalid address. Maybe I can’t do it with this specific router and its firmware, but I don’t know. Any help is appreciated.