|No, the point is maybe, that the modem has the ip address e.g. 192.168.1.1 and the router has the same ip address by default. In that case, one of both will lose, because the same ip address must be unique in a local area network.|
So if you get access to the router when modem is not connected, change the routers internal ip address from e.g. 192.168.1.1 to 192.168.1.2.
When finished you have to change the ip address in you browser to the new configure ip address. In this example, 192.168.1.2 and you should get a login screen.
Ok then. Now you have to connect the modem to the router, as it was before.
Check out, whether you can further connect to your router.
The initial requester should always write the last response !!!
Let us know, if the problem is solved !!!