EX6120 Problem

Hello everybody,first of all,excuse me for my bad english..i'm italian.
I have a little problem..today my internet connection stopped working,and opening the web interface i saw that there is no connection from the ap...So,i restarted the repeater,and the connection came back. After about 10 minutes the connection stopped working again,so i've done a factory reset and,after the site survey,selecting my ap appeared that translate into english is: This DFS channel is not allowed to use because the Radar was detected before. What does it means? And why the connection goes for only few minutes?

Additional informations can be that my ap is a cpe 510 of tp-link,i use the channel number 128 and everything was going good before today(no problem since the installation,1 month ago)..the last thing( i don't know it can be related to the problem) is that this problem appeared 30 mins before a big storm..