So if you are forwarding the external http port 80 to the machine port 83 then by default the built in http challenge service is on port 80 and you will need to change this by editing C:\ProgramData\Certify\serviceconfig.json and setting HttpChallengeServerPort to 83.
This still relies on whatever other http server you are using (IIS?) to be using the standard http.sys pipeline. If you are using an alternative http server (like Apache etc) you have to use filesystem validation instead as the challenge server will be blocked from listening for http requests.