Assuming that the Sonos device hardware is not faulty, maybe just see if the Boost has a wireless IP address (lease) conflict issue involving your router and see if simply rebooting the local Router will resolve your issue.

Check your router DHCP server later too and just see if the Boost is getting assigned an IP address whilst the Boost is not wired to the LAN.

Also give the Boost, say 5-10 minutes, in-between operating it in wired mode and wireless mode. If the problem still persists in wireless (SonosNet) mode, see what happens if you power it off/on and just bring it closer to the first Boost that’s currently wired direct to your router.

What happens if you simply swap the (problem) Boost with the original Boost that is wired direct to the router?

Important points - did you initially setup the Boost uncabled/away from your wired LAN, or did you wire it to the first Boost, or your Router? ...Did you use the ‘Add Product’ option in the Sonos App whilst all your devices were connected and showing in the App?

In its normal/intended location, how far away is the wireless Boost from your other ‘nearest’ Sonos product?

Are any Sonos product WiFi adapters currently disabled within your setup, as that may prove to be important too?

@mch831 

Wire the Boost again, then screenshot the network matrix at http://x.x.x.x:1400/support/review. x.x.x.x is the IP of any player, not a Boost. 

The radio cannot be disabled on a Boost (it would be pointless to do so) and the matrix will show the quality of its connections, and indeed if the radio has failed.