Coect to modem with a different IP range

my home network is coected to an Asus RT-AC88U router. The IP addresses are in the range of 10.20.30.X, with 10.20.30.1 assigned to the router.

On the WAN port of the Asus router it is coected the VDSL modem. The IP of the VDSL modem is 10.20.31.1

How can I coect to the modem from my PC? I try to enter 10.20.31.1 on the address bar but it doesn't work.

Do I have to tinker with the routing features of the Asus router? Can you please help me configure it?

Thank you Richard