Ethernet works from the modem but not from the router

[Owletta]

Ok so in my house we have what i think is a modem that goes into a router, if i connect my laptop to the modem via ethernet it works, and if i connect it into the cat cable going into the router it works but neither wifi or ethernet out works from the router. 
The router is a AC1200 “Smart WIFI Router Dual Band Gigabit.

the setup has worked for two years with only minor issues

[Levent]

Obviously that ethernet cable going to the router from the modem is the WAN/internet connection provided by your modem, if you remove that you will lose internet connectivity to the devices connected to the router.