I am going to try and sound nice here - these radios need to be installed by professional installers, and by ticking the agreement when you first log into them, you should know what DFS is.
From the ubnt instruction manual / wiki
Enable DFS: DFS is the part of the IEEE 802.11h wireless standard. Enable DFS option allows to enable/disable DFS support (applicable to M5 series only). DFS may be mandatory in some regulatory domains and should be tuned according to the regulations of the selected country. Please consult compliance guide and official regulations authorities for further explanation of compliance requirements for the country where AirOS v5.3 based device is installed.
Dynamic Frequency Selection will listen on the channel selected for a radar system - such as aeroplane tracking at airports, weather monitoring etc. A link can interfere with a radar system many many miles away. So when it starts up, it will listen for a radar and if it detects one, it changes to another channel.
To elaborate on what was said above - the DFS channels in many countries usually have lower output power limits, so that could be why the signal level has gone down - but its a big drop so im not entirely sure it is.