How to allow broadcast messages on a BR1 LAN

I use a BR1 to connect to starlink for my WAN on a sailboat… works great. But I also want to be able to reach my boat’s chartplotter which is a Garmin GPSMAP 1243. Garmin devices reside on their own network which is available via wifi, so I connect to it via a wifi-as-WAN link on the BR1. That works well too… I can ping devices on the Garmin network.