You’d need to connect over IPv6 (nc -4u is v4, I believe? ref).
If the agent is running on port 8125 in a Fly app named dd007 which may be running multiple instances (VM), say. You’d connect to it (from another Fly app / Fly maching running in the same 6pn network of the same org) either allocating and using a Flycast IP (over port 8125); or by explicitly connecting to any running instance of dd007 using its 6pn DNS name (on port 8125 as well) (1, 2), which should look like <alloc-id>.vm.appname.internal (a specific instance of that app in any region but same 6pn network and org), or region.appname.internal (all instances of the app in that region).
6pn has been finicky in the past: .internal DNS returns large number of invalid IP addresses