D-LINK DWL-6700AP. CONNECTION PROBLEMS ON THE 5GHz CHANNEL AT A DISTANCE OF 150 METERS

Hi to Everybody.
Recently I bought two AP D-LINK DWL-6700 because I needed to transfer the WiFi signal between two apartments located on the main street, not exactly face to face but perfectly visible one another. The distance between the two balconies where I put the devices is aprox 150 meters.
The two devices works perfectly on the 2,3 GHz and both create a powerful WiFi area.
Unfortunately the connection between them on the 5GHz doesn't work at all.
I set the same communication channel, apparently they start connecting each other but after few hours the connection is lost.
The 5GHz led power level on the devices is mostly 1 (red), or rarely 2 (orange), on a scale of 4, and never reaches the green status (3 or 4).
I tested the devices at a 1,5 meter distance and they work properly, with the 5GHz led power level always on 4 (green).
I tried all the different channels available for the 5 GHz connection (100 - 108 - 116 - 124 - 132) but the result was always the same, after few hours the connection was lost.
I contacted the D-Link Technical Support and they first suggested to upgrade the firmware to the last available review, I did it but no improvement on the 5GHz connection.
When the Technical Support suggested me to "ADD ANOTHER AP BETWEEN THE TWO TO REDUCE EVENTUAL LOCAL INTERFERENCES" I decided that I had to find the solution somewhere else.

Before returning the devices to D-Link I decided to post the question on your forum that I often read.

Thanks in advance to anybody who will be able to help me in this matter.

Paolo