In case someone else runs into the same issue, what worked for our team was to ensure that the client you're trying to access the server instance from is able to reach the server via the provided connection string and public IP address. A simple test is to run http://my.public.ip.address:6080/arcgis on your browser to see if you can get to the AGS instance without error. Some routers need to have loopback NAT enabled in order to load a resource through a public IP address while on the same LAN. Hope that helps.
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