no reverse dns in traffic analytics

Hey, 

I have enabled dpi on my edge router , which works fine, but all ip's aren't translated to hostnames via my dns service. I'm not using the build in dns server. I configured my dns server under system  -> name server . I think this should be enought , to resolve the hostnames. 

If I login to my edgerouter via ssh and  I can ping hostname without any issues. 

Any ideas ?

best ragards, sebastian