When you write that it is used as access point only is the WRT connected with the internet port or a numbered LAN port to the modem?
If it is connected through the internet port then you may be able to find out the IP address by wiring a computer to the router, opening a command prompt window and entering "ipconfig /all". Look for the Local Area Connection and check the entry for the default gateway or the DHCP server. Both should be identical and maybe something like 192.168.2.1 or similar. The gateway/DHCP server address should be your router.
If the router is connected through a LAN port it is more difficult. In that case the router should not run a DHCP server thus you can only guess what the IP address might be. Try http://192.168.1.2/ or http://192.168.1.254/ Or enter "ping 192.168.1.2" into a command prompt window. Try different addresses. You can only try 192.168.1.*.
If that does not work or takes too long get an IP scanner like http://www.angryziber.com/ipscan/ You can only scan your current subnet, thus if you computer has an IP address in 192.168.1.* with a subnet mask of 255.255.255.0 you can only scan 192.168.1.1-254. Make sure that the scan works properly. You should at least find the modem at 192.168.1.1. You may scan for an open port 80, too.
If you cannot find the router like that it gets more difficult. The router can use any arbitrary LAN IP address but you will only be able to find something with the IP scanner if your computer is in the same subnet as the router. As you don't know the LAN IP address you don't know the subnet thus you could only try one after another. I guess it would be easier in that case if you simply reset the router pressing the reset button for 30 seconds and set it up again.
The only alternative would be to use the IP scanner to scan larger IP subnets, e.g. 192.168.*.* and check the arp cache periodically ("arp -a" in a command prompt window) to see whether the router appears there at some time. The IP scanner itself won't find it but the arp cache would reveal it.