Load balancer behind Network Load Balancer
Can not access layer 7 application load balancer backends via layer 4 network load balancer.
In a VCN there are two subnets, public and private, both containing compute instances.
A (layer 7, application) Load Balancer has a backend set with two backends (compute instances' IPs) and a listener on port 80. The LB and backends are in the same private subnet. The instances are running httpd. The load balancer, its backend sets and backends all have OK health.
The load balancer's endpoint can be used successfully from instances in the public and private subnets.
A Network Load Balancer in the public subnet has a backend set with one backend, port 80 on the private IP of the private subnet's load balancer. This remains critical and the backend set remains offline despite its target, the private load balancer, being in full health and accessible from the same subnet where the network load balancer is. The TCP health check points to port 80.