MVC 4 on server works, blank 200 via internet

I have an MVC 4 site on Windows Server 2016. If I am on the server and point a browser to localhost, the site works fine. If I point a browser on the internet to the http (or IP) of the site, I get a 200 and content length 0.

I've tried to access a gif from the site and get a 404.

I've found nothing in Event logs and the IIS logs does not register any hits to the site.

I've Googled this extensively and am none the wiser.

Can anyone suggest what I can check next?