How to configure a route on an iMac's Wi-Fi interface?

Question

I am terrible at network routing.

I have an old Squeezebox SB3, whose Wi-Fi card failed some years ago. I removed the WI-Fi card, and it has gathered dust for a while. I am trying to resurrect it.

It no longer responds to the remote control. But you need to use the remote control to reset it. The remote seems to work (I pointed the IR diode at an Android phone's camera and I can see light when I press any of the buttons).

Because it is not at the factory defaults I cannot use Net::UDAP.

So the current setup is:

SB3 connected via an ethernet cable to one of the ports on my cable modem.

iMac connected to the same cable modem via Wi-Fi on 192.168.1.0/24.

Logitech Media Server (LMS) is running on the iMac and is happy to bind to any IP address that is configured on the iMac.

tcpdump tells me the SB3 is sending out packets like this periodically.

20:37:30.123202 ARP, Request who-has 192.168.10.1 (Broadcast) tell 192.168.10.10, length 46

From that I deduce the SB3 uses 192.168.10.10 as its ethernet IP address and expects to find LMS running on 192.168.10.1:9000. I used ifconfig to add 192.168.10.1 to en1 (see below), and I can open a web browser on the iMac and load LMS on 192.168.10.1:9000. I can also open 192.168.10.1:9000 on a web browser on one of the household smart phones that are running on 192.168.1.0/24.

But the SB3 is not seen by LMS running on the iMac and the SB3 cannot find LMS either.

bash-3.2# ifconfig en1
en1: flags=8863<UP,BROADCAST,SMART,RUNNING,SIMPLEX,MULTICAST> mtu 1500
    options=400<CHANNEL_IO>
    ether xx:xx:xx:xx:xx:xx 
    inet6 fe80::144f:39e8:91fa:1f29%en1 prefixlen 64 secured scopeid 0x4 
    inet 192.168.1.23 netmask 0xffffff00 broadcast 192.168.1.255
    inet 192.168.10.1 netmask 0xffffff00 broadcast 192.168.10.255
    nd6 options=201<PERFORMNUD,DAD>
    media: autoselect
    status: active

on the cable modem I added a route

192.168.10.0/24 IP_BR_LAN 192.168.1.23 AUTO STATIC

I suppose I have a routing problem, and I don't know which routes to set up on which device (iMac or cable modem).