ok, then you can also change ipaddress on the system, i’m not familar with openwrt but temporary you can change it like this:
ip addr del 192.168.1.1/24 dev lanbr0
ip addr add 192.168.1.200/24 dev lanbr0 // or something else
and then connect any of the lan-ports to you existing lan (so you have same subnet). for internet-access you have to set default route and dns-resolver
ip route add default via 192.168.1.1
dns-resolver depends on what openwrt is using, i guess @dangowrt can help here. debian/ubuntu uses /etc/resolve.conf or systemd-resolved
then you can install lucy and make all permanent network config there