Home Network :: PCs On Modem Can't Find PCs On Router?

I have a wireless modem on the first floor of my house, where I got cables through the wall running throughout rooms in first floor, ground floor and basement.I have my small business at the basement, so I have 2 PCs one printer and one external hard disc which also connects directly to a modem/router.I have 3 PCs from the first and ground floor that connect through the wall to my modem, and everything works well. But for my basement, which only has 1 lan cable output on the wall, I got myself a router so that I can connect all my devices.I connect the router with my modem, and then the PCs, printer and hard disc on the router. The PCs have internet access, so I know the connection to the modem has nothing wrong to it. But I am not able to access the PCs connected to the router from the PCs connected to the modem, or vice versa.

I was told to not obtain an IP address automatically, so I assigned my modem and router to work on 192.168.10.x and gave my PCs IPs such as 192.168.10.19 Only after setting this up did all my PCs have internet access, but again, no network communication between PCs on modem and PCs on router.One thing that might be worth mentioning, is that through time, my network keeps going from Home Network: Network, to Network 2, Network 3, and so on... now it's on Network 5. I have no clue why this happens, and if it's related to my problem