Using an 18.3 AMP appliance over 14 gauge wiring
No. No no no no no.

The sole purpose of the breaker is to protect the wiring from overheating. It's not just there to be a buzzkill. If you use a breaker that is too high for the wire you will lose that protection and there is a very real possibility of overheating and starting a fire.

An 18.3 amp device needs to be on a 20 amp circuit, meaning the receptacle, wiring, and breaker all need to be rated for 20 amps. (That would mean 12 gauge wire or bigger.) By the way, is your sauna really 120V? I thought most of them were 240V, which of course is a whole new thing.