You have a router running in your network which is connected to internet. You want to connect second router/switch in your network thru Ethernet cable. And you want to access devices connected to both routers from any router then follow below steps.
Unplug the second router/switch from network. Connect computer to the router/switch (Do not connect the second router to the first)
Configure the second router/switch at http://192.168.1.1/ (or whatever the default IP is)
Change IP address of the second router/switch from 192.168.1.1 to a free address in your LAN (e.g. 192.168.1.2). The address you change to (192.168.1.2) must not be used by any other device in your network nor should be assigned by the DHCP.
Turn off the DHCP server on the second router/switch and save settings. Reboot if necessary.
Unplug the computer from the second router/switch.
Connect an Ethernet cable from LAN port of the first to LAN port of the second router. Do not connect to the WAN or Internet port of second
Why second router not connected to WAN / Internet Port?
So that all devices connected to any router will remain in same broadcast domain. This results in devices connected to both routers to access each other without doing any additional route definitions and some manual configurations. Otherwise computers connected to the second router can connect to computers connected to the first router but not in the opposite direction.
In the configuration defined above, router part of second router is not used, it only work as a switch. Devices connected to second router will actually take IP address from DHCP of first router. Everything is connected to a single larger ethernet network. Everything is in a single “broadcast” domain.