Hello, if you are referencing the proxy page proxy.ashx then you will want to use the IP address option on the get token page http(s)://[webserver]/ArcGISInstance/tokens/gettoken.html
From there ***warning I am not a network guy***
I had ping'd my outside domain and got back the internal ip address of my outside domain, and that is the ip I had to use in order to create my token
Meaning, I had to use the ip address I get when connected to my network, versus the ip I get when ping'ing my url for a computer not on my network
What are you using to generate your token? Are you not using the token page?