After creating a firewall rule to allow SSH ingress traffic from frontend to backend instances, I’m unable to actually gcloud compute ssh to the backend instance

I have created a firewall rule that should allow all ingress traffic from frontend to backend instances on port 22, so I can ssh to a frontend instance from Cloud Shell, and then ssh to a backend instance from there, theoretically at least. I know about bastion/jump hosts, and this seems to align with that concept (correct me if I’m wrong). But when I actually try to ssh from a frontend instance to backend I get the following error:

[email protected]:~$ gcloud compute ssh backend-instance-group-rm88 –zone us-west1-a

ssh: connect to host 35.203.145.51 port 22: Connection timed out

ERROR: (gcloud.compute.ssh) [/usr/bin/ssh] exited with return code [255].

Any suggestions? I’m kind of stuck.