Insurer denied burst pipe claim as 'gradual damage' - pipe literally burst overnight. How do I fight this?

I woke up on a Saturday morning in February to water pouring through my kitchen ceiling. A pipe in the wall had burst overnight from the cold - we had a temperature drop to 9 degrees. By the time a plumber got there the damage was significant. I filed a claim expecting the process to take a few weeks. Instead my insurer denied the whole thing and labeled it "gradual damage or seepage" which my policy excludes.

I have a plumber's written statement that this was a sudden pipe failure caused by the freeze. I have photos timestamped the morning it happened. I have a neighbor who also had pipes burst that same night. And they're calling it gradual. I'm furious. The repair estimate is $18,400 and they want me to just absorb it. What are the actual steps to fight a denial like this? Is a public adjuster worth it here, or should I go straight to an attorney?