Activity Monitor Blocked by Firewall. Set Static Port?
Activity Monitor requires the firewall to allow access to the SQL Server port (default 1433) for SQL query access and the RPC ports for WMI access for things like the Processor, Network, Disk IO and Memory graphs. This blog describes this and the required permissions for viewing all of Activity Monitor.

This article describes how to configure a static port range for RPC on a machine to allow you to open those ports on the firewall, otherwise, you have to open a substantial range of ports. Due to the number of applications that can rely on RPC ports, you shouldn't restrict this range to a single port, but instead, use a range size suitable for the number of applications and that your security team is comfortable with opening on the firewall.