they may have the same address, try plugging just 1 in at a time and setting the address to something different
i had to do this with my WRT + trendnet, wrt is set to 192.168.1.2 trendnet is set to 192.168.1.1
Basically change the LAN/Router IP on the 2nd router to +1 of your main router. So like the post above, main router = 192.168.1.1 and the 2nd router = 192.168.1.2.
Then disable DHCP on the 2nd router. This way any devices connected to it will pull the IPs from the main router and everything will be able to communicate with eachother.
And to wire it up, you'll want to avoid using the WAN port on the 2nd router (unless you change the defaults in the configuration, which I'm assuming Tomato can do). Just use the LAN ports only, so R1-LAN1 connects to R2-LAN1.
If everything on the LAN is using DHCP then you want to assign the WRT54 an IP address that's outside of the DHCP address pool and not used by the WRT150.
But if all you want to do is to change the firmware then easiest way maybe to just physically unplug a system from the LAN, connect that system to the WRT54 and do the upgrade. I usually use my laptop for such tasks.
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