I am not having the same setup, but I had a similar problem. My FireFly was sitting on the ethernet side, but both soundbridges and also iTunes where on WLAN. Problem was, that after a few minutes/hours the FireFly did disappear from the WLAN side of the router.
I did traces and stuff and finally found the issue to be the router itself. I have tried Zyxel, D-Link and Netgear, all had the same problem. I now switched to Linksys and the problem disappeared.
You can also check if your router has something called “WLAN isolation”, which will prevent two WLAN devices to see each other. Try turning this off. Otherwise, your only help is to replace the WLAN router.
Ron