Custom DNS on kUBUNTU 18.04.2

I wrote you this because I tried to use KUBUNTU with QUAD 9 DNS, CLOUDFAIRE OR OTHER DNS, BUT I DO NOT WORK I TRYED WITH REZOLV.CONF WITH CONFIGURE THE NETWORK CONNECTION UNDER KDE WITH AUTOMATIC (ONLY) WITH STUBBY. ALL WORK AFTER REUSE, BUT AFTER A FEW MINUTES, REGISTER ON THE ORIGINAL DNS RENDERED BY MY FAI.
BEST WISHES!
Sorry for my bad English, there are some.