[mothered]

When I asked for clarification or to be escalated, they told me nothing more could be done.

Nonsense, It's not up to them to decide.

You have every right to request your claim be escalated and reviewed by a senior agent who was not Involved In the original decision to decline It.

The only suggestion they made was to call their specialist team myself, but they would not refer me. I plan on calling them.

Do exactly that and reply when you receive their response.

Also, request the evidence that clearly demonstrates why the claim was declined.