If you do not even get to a Kentico screen, then nothing to do with Kentico.
How normal setup works with IIS is that a Domain points to an IP address, which points to the IIS server.
IIS receives the requested domain, and using it's Bindings maps one of it's site to it. That's how you can host multiple sites on one IIS server, say "Test1.com" and "Test2.com" they both may resolve to the same IP address, but when the IIS server gets "Test1.com is asking for a site" it knows which ones to provide.
If your Proxy is redirecting IP addresses to the IIS server, you need to look at what "domain" so to say it's receiving. Note that an IP address can be the domain, but then IIS needs to know that that single IP address needs to be bound to the one kentico site, and you'll have a hard time hosting others unless you add some Port to the IP address (such as say 192.168.1.1 is your IIS server internally, 192.168.1.1:89 => Test1 site, 192.168.1.1:90 => Test2 site)
If you can remote onto the server, you can use WireShark to test incoming requests, to see what your proxy is sending to the server.