Convert a router to a switch?

I have a couple of modem/routers that came with ISP services that are no longer being used. At the moment I have the current ISP's router running the wireless network and a wired portion to a couple of rooms. In one of those rooms I have a switch to connect all the computers there to the cable coming from the router.

In the other room I have a couple of cables (actually one split into two) but I need at least one additional wired connection there.

Can I convert the existing routers that I have to behave as switches? I don't want these routers to control their own subnet giving IPs to devices connected to it. They should be as transparent to the network as if I just had a few additional wired connections available there.

That very much depends on the routers.

If you can disable the DHCP server on the router and the router has a built-in switch then yes, you should be able to use it without problems.

If you can't disable the DHCP server then It'll cause problems on your network, so don't use it.

• Disabling DHCP is one step
• The next step is not using the typically upstream connected WAN port and using only the LAN ports for upstream (connect to router etc) as well as downstream (clients) will cause the LAN ports to behave as as switch and then it would need to pull DHCP IP from an upstream device such as Consumer/ Internet Router or DHCP Server (on a NAS or Domain Controller).

For E.g., In this Home Media Network below the Router WR1 is a DHCP provider for the Local Subnet while the Router WR2 is connected via LAN ports with WAN disconnected. That way they are all on the same Subnet and all IPs are managed via WR1 (Tomato USB firmware based Router)

If I was to connect WR2 using the WAN port (with its own DHCP enabled) it would create another Subnet of its own.