Application Guard requires proxies to have a symbolic name, not just an IP address. IP-Literal proxy settings such as 192.168.1.4:81 can be annotated as itproxy:81 or using a record such as P19216810010 for a proxy with an IP address of 192.168.100.10. This annotation applies to Windows 10 Enterprise edition, version 1709 or higher. These annotations would be for the proxy policies under Network Isolation in Group Policy or Intune.