Need to disable the modem in my modem/router

[monkeygurl]

So I have a modem/router. To get internet I have another modem from my isp. I need to disable the modem from my modem/router for it to work as only a router not both. I am a tad tech savvy so I may need specific details but I can login to my modem/router and know where the command prompt is

 

[The Paladin]

make and model of your combo-modem/router?

 

[monkeygurl]

A Zyxel C1100Z

 

[The Paladin]

your manual is located at https://www.manualslib.com/manual/1272589/Zyxel-Communications-C1100z.html?page=7#manual

I would look in advance setup, but I saw nothing to turn the modem off.

 

[monkeygurl]

I haven't found a setting to turn the modem part off. But I have been trying to find a way to just use the router and disable or tell it to work around the modem part of the modem/router.