This is a repost of posting #632544 (very old), but is exactly same issue I am running into now.
What Works: I purchased 2 Arduino UNO WiFi Rev 2 units. The boards both work great when they are powered by the USB port. I am able to upload sketches and run them with ALL functionality including WiFi connectivity.
PROBLEM: However when I plug in any 9-12 volt supply and remove the USB connection, the UNO WiFi Rev 2 boards will not function. A Network scan shows they aren't even connected to the network. The green onboard power LED is on. Pressing the reset button does not change anything.
Anyone have any insight to this issue?