At first I was tempted to mark your question as a duplicate of this one. The answers there give useful hints on what to do, but in my opinion their educational value is low in a field of explaining why.
Your DD-WRT router probably acts as a switch.
In my workplace we have several home routers in such mode, their purpose is to distribute Wi-Fi and split wired connections. Even with their basic (non DD-WRT) firmware one can do this by:
- disabling DHCP,
- using only LAN ports for wired connection,
- setting up the device (LAN interface) IP to something that doesn't collide with anything else in the network.
Your DD-WRT router may have stopped its NAT service, added its WAN interface to the bridge (that normally concentrates LAN and Wi-Fi interfaces); it's not really important. The above rule of using only LAN ports is solely because mediocre firmware cannot do this. By not using WAN port we can bypass this limitation.
The important thing is: a device in this mode seems transparent to IP traffic. It usually has some IP address assigned but the address may or may not be from within your local network. The root of the problem is you don't know the address nor the netmask.
If your other router provides DHCP for your LAN and if your DD-WRT router uses it do set its IP, the DHCP lease table on the other router will tell you what the IP is. Check it out if you can. However the DD-WRT router may not use DHCP, its configuration may include some fixed arbitrary IP.
You can scan your network and try to find the right IP. See this answer. While scanning some address range or trying to reach the web interface of the router, you need to set your own IP address and netmask in a way that:
- your computer considers scanned addresses as local;
- DD-WRT considers the IP of your computer as local, according to its own IP and netmask you don't know;
- both devices have different addresses.
In practice this means you cannot scan large set of addresses at once. For example you may expect the IP of DD-WRT router to be like
192.168.*.*. To scan the entire range you set up your computer to, say,
192.168.2.5/16, this way the entire range is your local network. But if the router has
192.168.3.10/24 then it won't be able to reply because its netmask narrows down its local network. In this particular case you need your computer IP to be like
192.168.3.*, even if its netmask is
In theory your DD-WRT router can have almost any IP assigned. Even if it hijacked my external IP, the router could act as a switch for you and not interfere, unless you tried to communicate with my router. Your router may also have extremely narrow local address space. I suppose it may have no IP at all (in theory; in practice very unlikely, because this would render the device unmanageable, nobody wants this). Because of this you cannot expect for sure your scanning attempts to succeed eventually; you can only hope they will. The reasonable approach is to scan the most probable address ranges separately (
If I were you I would sniff the network with
wireshark. There's a chance some daemon or whatever on your DD-WRT router sends network packets that would reveal the IP address.
If everything else fails, DD-WRT provides a way to reset the configuration to defaults. It looks like you router needs to have a reset button, I hope there is one. After you reset, set the router up as a switch anew, noting the IP address for future reference.