I have a router which acts as LAN repeater, but I don't know its IP. If I tracert my firewall (gateway), it won't show my wi-fi router. Question: How can I find it? I tried to nmap all my network looking for an open HTTP port, but nothing. I also tried to unplug the cable coming from my firewall, but It doesn't have DHCP set.
When I have a device with a static IP address that I've forgotten, I usually plug it directly into my laptop via Ethernet (with a crossover cable if your laptop doesn't do auto-crossover/auto-MDI-X), then run tcpdump or Wireshark or another packet sniffer, and then power-cycle the device.
On boot, most devices will use ARP to make sure their static IP address is not in use by someone else, before using it themselves. So you can look for those ARPs to see what IP address the device is trying to use. Also, devices that do peer-to-peer name resolution and service discovery protocols will often make a few announcements or queries/probes at boot, so look for those packets too.
Make sure to use a filter in your sniffer to filter out any traffic your own laptop is sending. You just want to see what the device is sending.
Kinda sadly, the best tool I have ever found for network discovery is the network mapping tool from Windows 7 (AKA LLTD). Nothing else I've used comes close, and it will even show you switches and other devices without an IP address. Here's an example where it was the only tool that could even detect a specific device on a network.
There's a plugin for zenmap that does the protocol, but it seems nowhere as complete as the Windows 7 version of it. There's also a Linux scanner that does it here, but I've not had much luck with it either.
You haven't really mentioned what OS you're on. You shouldn't be just looking for a open http port. Many routers use https or non standard ports. Sometimes people just turn it off. That said - nmap (or zenmap) should be giving you the information you need.
If all else fails, consider resetting it, and setting it up again. It would likely be quicker.