As mentioned, if the nests are big and plentiful, you should consider a
pro. If not, you have to hit them when it's cool. When it's warm the
hornets are out and about -- when it's cool, they're typically at the
nest and move slow.

I haven't used a fogger-in-a-can, but the spray I'd recommend is TAT.
You can get it at MAB or Sherwin-Williams [I forget which], or Google
it and try to find a retail outlet. It's the one spray that's truly a
one shot kill.