Connect to a second wan but not route any traffic


I have a pretty standard OpenWRT setup with a WAN and a LAN.

I would like to connect my router to a second external LAN. The reason for this is that I would like to use some of the devices (mainly printers) connected to this LAN, I do not want to route any traffic towards this LAN in order to reach the Internet. The LAN provides me with an IP address over DHCP.

Can I configure an interface in OpenWRT to claim a DHCP address for a specific interface but prevent it from setting up routing for more than the actual /24 LAN that I want to reach?

Depends on the router, but yes you can.
Add a new interface on your router with dhcp protocol and assign it on a physical interface which doesn't belong to lan anymore. This can be done with a new vlan on the switch configuration. Also select to ignore the default gateway.
Then assign this interface to a new firewall zone and allow traffic from lan zone to this new zone.