After provisioning your P300 bridges, did you configure the Distance between them?
From Configuration -> Wireless -> Radio 5G page, click Edit Advanced Settings.
Set the approximate Distance (in Km), where the default is 3Km. If you have only 120 meters, use 1 Km setting.
Click Update Settings to confirm your changes. Do this on both sides, as Distance field affects timing. You might
get better than 285 Mbps throughput.