Testing Ubiquiti gear
It's probably best to test Ubiquiti gear by flashing it with the newest official firmware via tftp, then ensure that it works as expected before putting OpenWRT on it.
The newest AirOS can be downloaded from Ubiqiti's website.
See the notes on how to flash on the Bullet M5 section of the OpenWRT page.
After flashing with latest AirOS, the Bullet M5 did not come up with an IP in the 192.168.1.x range. Rather, it came up with the IP 192.168.250.52 and the username and password was not the default. This is because reflashing the Bullet M5 apparently does not reset its settings.
In order to reset settings to factory first wait for the router to boot (give it a couple of minutes after plugging in), then press and hold the reset button until all of the LEDs light up (about 10 seconds) and release the reset button, then wait for a couple of minute while the router reboots. The Bullet M5 should now come up with IP 192.168.1.20 and username/password will be ubnt/ubnt.