POST Request taking 90+ seconds?

I'm using an ASP.NET Core Minimal API w/ RestSharp to facilitate an OAuth 2.0 process with an IdP provider. I am sending a POST request via a web app and get a successful response... after about 90 seconds. The provider support team can't see logs, and I do not have browser developer tools, a debugger, or Postman equivalent. Ridiculous, I know. The only other bread crumb I get is a "Failed to gracefully shutdown application" warning in IIS. However, app still runs with no exceptions after request is complete. Timeout is set to 5 mins. Any ideas what could be happening? I'm suspecting maybe a firewall issue or IIS issue. At my wits end with this so any suggestion helps.