Issue turned out to be a DNS issue and our search head was not able to resolve DNS names for the indexers in AWS. As an interim solution we have updated the hosts file on the search head with the AWS pool of IP addresses and hostnames for the AWS indexers. Not elegant by any means but is temporary until DNS forwarders can be set up.
we are using DNS names on all configurations and updated those when creating new server / after termination , when server brings up with different IP. This has done on our ansible scripts by calling r53 services. Is this suitable option for you?