I have the next config:
PC -> Asus RT-N16 router (192.168.1.1) -> LinkSys WAG54G2 in bridge mode (192.168.0.1)
How can I access to modem's config page from my PC?
Just enter your modem's IP address. Your computer will find out it is outside your current subnetwork (given this is configured to 255.255.255.0) and thus will send your request to your internet gateway, the Asus router. This will route all internet traffic to your modem (which actually is also a router in your configuration).
If you cannot access your modem's config page, "internet access" (which it is) could be disabled for configuration page. Usually routers decide by network port whether a page request comes from within or without your network, but it could also check your IP address.