I should have tried to use a normal IP network switch or the hardware
switch part of a router. Connect first the Radar scanner and M121 to the switch and check if it works.
If you use a switch M121 has to have an own IP address. If M121 is able to get an address from a DHCP you can use a router set up for that. The scanner don't care, it can connect to either of. When you have a normal connection to M121 you can connect also your PC to the switch. Then the M121 will be the radar controller for range etc. If the BR24Radar_pi is activated the scanner picture will overlay your OCPN charts
. You may try to click the BR24 icon to get the plugin to find the scanner. I've tested something similar using a router with DHCP.
The IP addresses are not important for the plugin function since the scanner broadcast via UDP but a switch may be squeaky.
You should not in first hand try to control the scanner from your PC so set the range in BR24Radar_pi to some manual value.
I may babble but these are my ideas. If I have got a M121 I should have tried something like this. It can either work or not, you shouldn't be able to destroy anything.
Have fun - Håkan
(Hils Lista. I visited Farsund this summer, not so far away.)