You have two “easy” options: 1) install an extension flange (this fits inside the lower flange and raises its flange to just above the floor level); or, 2) use a “deep” wax ring that has a plastic sleeve that extends about 1 1/2 inches below the toilet outlet to allow a good seal with your recessed flange. A third “more difficult” but in many ways “better” option would be to cut out the current closet bend/flange and install a new one that is correct for the installation. BTW, how did you end up with a flange below the floor level? Did you add a subfloor and floor covering, like tile? Or was the original installation screwed up?