When importing non-OCP clusters in RHACM, their server API URL isn't getting populated, see this bugzilla for reference.
To workaround this issue, you need to apply the infrastructures.config.openshift.io CRD in the managed clusters, and then create their Infrastructure CR instance providing the server API URL. This is where the RHACM appmgr addon is getting the information.
Note: the CRD was taken from an OCP 4.9.23 cluster, and I'm using RHACM 2.4.2
In my environment I had to do the following
ca-toronto
kubectl apply -f infrastructures.config.openshift.io-crd.yaml
kubectl apply -f infrastructure-us-philly.yaml
ca-toronto
kubectl apply -f infrastructures.config.openshift.io-crd.yaml
kubectl apply -f infrastructure-ca-toronto.yaml