Make sure the router is not using the same subnet mask and gateway as the modem otherwise you'll probably run into a conflict because the system will not be able to distinguish the difference between the devices.
Example of a correct configuration for a home network:
Router IP: 192.168.0.1 Subnet Mask: 255.255.255.0
Router Lease IP Address Ranges: 192.168.0.100-192.168.0.199
Modem IP: 192.168.100.1 Subnet Mask: 255.255.254.0
So if your modem is 192.168.1.1 and your router is manually set to 192.168.1.2 you are on an IP address that should be leased by the modem automatically to the router. Technically the IP address gateway of the modem should lease all addresses from ranges 192.168.1.2-192.168.1.254 but in this case it shouldn't need to because you only have one port on the modem unless it's a combo modem+router. So if you try to log into the modem using 192.168.1.1 with your router set to 192.168.1.2 it will instead look for a device connected to the router because your PC is currently connected to the gateway 192.168.1.2. Basically don't use the same gateway of the modem for the router i.e. 192.168.1.xxx, instead change the gateway of your router to something like 192.168.0.1 so when you enter 192.168.1.1 and your computer is on gateway 192.168.0.xxx leased from the router from 192.168.0.1 it will know to look for a different gateway. Confusing right?
Hopefully I explained that correctly. I had trouble many years ago first learning how to network properly understanding how gateways and subnet masks work. Before manufactures would use the same gateways for the modem and the routers and if you weren't knowledgeable about networks your internet connection wouldn't work after you put a router in between your PC and the modem. But now most manufacturers usually try to not use the same default gateway/subnet mask for the modem and router so you don't have this problem.