how to set GCP LoadBalancers with custom port for RMP nodes?

Hi, I need a bit of help regarding LB in front of RMP nodes on GCP.

I configured 5-node topology on GoogleCloudPlatform. There are 2 RMP nodes and I want to configure an External LB for client apps and an Internal LB for internal traffics using the GCP TCP LB service.

However external/internal LoadBalancer did not work as I expected. The response was “Connection Refused” when I requested the LB IP address.

It worked fine on AWS under the same conditions. Is there any guide or reference to configure the LB on GoogleCloudPlatform?