Ports are locked down in IIS Express so that it doesn’t have to be run as Administrator…
Valid Ports are 44300 – 44399
Check out the Dev Community article
https://developercommunity.visualstudio.com/content/problem/39430/changing-port-number-in-a-web-project-does-not-imm.html
You can edit launchSettings.json, but the ssl ports must fall in this range.