I suggest reading these articles - https://cloud.google.com/compute/docs/tutorials/how-to-configure-ssh-port-forwarding-set-up-load-testing-on-compute-engine/ and https://www.smartmeter.io/documentation#toc-controller---generators-communication for better understanding of communication between nodes in distributed mode.
The hostnames you are setting via -Djava.rmi.server.hostname, are they hostnames of the machines the instances is running on? I mean 10.168.88.2 is the hostname of the server and 192.168.88.2 is the hostname of your Ubuntu machine?
Also make sure you can ping from the server to the client, because server needs to connect to the client in order to send results.
If you cannot make direct access from server to client, use SSH tunneling (described thoroughly in the first article).
Hope this helps.