How to use a modem with a modem+router combo ?

Factual Questions
1

I tried to google this but all I get are explanations of the differences between a router and a modem :frowning:

Anyway, I have a DSL modem plus router combo where the modem is not functioning properly but the wifi router is fine. I want to purchase a new modem-only unit and then use just the wifi/router part of the existing modem+router unit. Is that possible ? How do I configure the modem+router to function as only a router ? The existing router is a TP-Link (model td-w8961nd).

thanks !