GitLab HA Runners

After a bit of help / guidance in setting up runners with a GitLab HA environment. We are rolling out a 20 server HA cluster. This is up and running. The issue I have concerns registering runners to the cluster. I put in the URL to the server and the other bits but it always errors at the end doing the final part.
One thing that springs to mind is that we are using a load balancer (HAProxy) in front of the application nodes. This could be preventing the runners from actually connecting.
Has anyone got an experience with connecting runners to an HA cluster and any pointers on things to look for. Are there any additional ports that need forwarding from the load balancer etc that could help.
Thanks in advance.