How can I access the GUI of my router?

Episode 1123 (1:13:15)

Rick from Albuquerque, NM
Asus Router

Rick bought an Asus router and set the cable company's modem/router in bridge mode. He wants to know if he can access the GUI of the modem because he can't see it. Leo says that's because the Asus router is assigning the IP addresses as it goes. That's how it's designed. So he can't really do that. Leo also says he could alter it via the public address by turning on WAN administration, but that's a bad idea because then anyone could do it.