I have seen this a few times primarily with Comcast clients. Comcast says its not them.
If i configure a network resource to be accessible via the internet I can access them via static IP address or domain name pointing to that network via any 3rd party isp connection. However if I try to access those devices using the same IP addresses or domain name from a computer on that same network i cant get to it. I have to used the local Ip addresses.
In the past I have found work around with remote desktop connections or ip cameras. ( make two different shortcuts) but Now I have two laptops configured for IMAP to a local exchange server. Outlook works via 3rd party isp but not when the laptop is at the office on that same network.