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.

tp-link 192.168.1.1
backup router 192.168.1.2

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
81.174.0.21     *               255.255.255.255 UH    0      0        0 ppp2
192.168.1.0     *               255.255.255.0   U     0      0        0 br0
default         *               0.0.0.0         U     0      0        0 ppp2
 > route delete default 0.0.0.0
Invalid destination IP address
 > route add default 0.0.0.0 192.168.1.2
Invalid destination IP address
 > route add 0.0.0.0 0.0.0.0 192.168.1.2
Please at least enter gateway IP or interface

I also tried to add it via the HTTP interface, but it tells me that 0.0.0.0 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.