Probably NAT, or overlapping subnets. On whichever device is not connected to your modem:
- change the management IP to something unlikely to conflict with other devices (192.168.1.254 would be ok with the default settings on both routers)
- turn off DHCP on that device
- connect the uplink to a LAN port (not WAN)
- have devices behind that router obtain a new IP lease, rebooting will usually do it (devices with a static IP may need to be reconfigured)
This will put your devices on the same network segment so they can communicate with each other.