@Edo5287 

 Your statement is confusing.

Edo5287 wrote

I specify that my friend has a modem with a single 2.4 and Ghz Wifi network.

I sounds like you are saying it is only 2.4ghz but then you say "and" , which sounds like it may also be 5ghz. That needs to be clarified first because the CPE710 only works on 5ghz.

Are you using one or two CPE710?  If your friends modem is only 2.4ghz, you will need two CPE710.

  @RickJamesBish I was wrong. I meant that my friend has a unique 2.4Ghz and 5Ghz network.
Later I'll try configuring the CPE 710 again, making sure to place it as close to the modem as possible.

  @Edo5287 The ideal solution would be to buy two CPE 710s, but my friend doesn't want to.

  @Edo5287  you also mention a distance of 10 meters, which unless I am over looking something, is only 32 feet. 

I have a similar setup with one CPE710, linked to an AP that is about 25 meters away but the AP is outside. I get -32, which is practically 100% signal. I can turn the CPE facing over 90 degrees away from the AP and still get -60.

Based off your comment about accessing the 5ghz with your phone but it is very weak, I am guessing the modem is either behind multiple walls, is already very weak, or both.  5ghz has very poor penetrating characteristics. The problem multiplies for every obstacle between the host and the client. If your friends modem is inside and has several walls between it and the client outside, that will most definitely be a problem.

Things to try

1. Check the signal strength and performance of the 5ghz channel on the modem. Check your signal strength just feet from the modem. Move to another room so there is a wall between you and the modem and see how much the signal degrades. If it is noticeably bad, that needs to be addressed first. If the signal is that weak inside, it will suffer horribly outside.
    
2. Check the channel width for the 5ghz radio on the modem. It should have options for 20, 40, 80 and maybe even 160.  If it is at 80 or 160 that can most definitely be part of the issue, especially with any obstacles between the modem and client. Try 40 or even 20 to see if it improves the signal outside.
    
3. Move the modem to an exterior wall facing your property, preferably a window.

  @RickJamesBish Thank you for your answer.
I confirm that from my house to the access point there are about 10 meters, with a wall in the middle (friend's house)
Yesterday I quickly retried from my window to my friend's access point with -75db power. Could it boost the signal if I put the 710 on the roof?

  @Edo5287 no not really. The signal the 710 is picking up is weak so you will just be picking up the same signal but from the roof. You really summed it up when you said the best thing would be to have two 710s.  The only other thing I can think of is see if he is running 80hz on his 5ghz radio and if he is put it on 40hz or 20hz and see if it helps.

  @RickJamesBish If I buy the second 710, and install it inside my friend's house (near a window), will it be ok? Or does it necessarily have to be hung outside?

  @Edo5287 I have never tried so I cannot say for sure but any obstruction is going to have some negative impact.

  @Edo5287 Thanks for your help. At this point I will have to hear from my friend, to propose him to install 710.