Private Last Mile
Private Last Mile servers are owned by the Private Last Mile customer, so you should be able to get the IP address from the customer.
Last Mile servers are owned by end users and therefore are out of Dynatrace control. Last Mile tests run on the machines of individuals who have signed up to install our agent and let us use those computers for testing. Thousands of Last Mile peers may be online at any time, and the IP addresses for these machines could change daily.
You could restrict access to your website by allowing a request through that contains a specific User Agent string or request header — i.e., blocking any request that does not contain the User Agent string or request header. This would have the same result as filtering IP addresses.