[Satellite 6] Verify return code: 21 (unable to verify the first certificate) when using 'subscription-manager register'

Issue

The customer is able to successfully run the following command:

# yum localinstall http://<satellite_FQDN>/pub/katello-ca-consumer-latest.noarch.rpm

It appears to place 3 files in the following directory

[ ~]# ll /etc/rhsm/ca/
total 12
-rw-r--r--. 1 root root 7732 Sep  1 10:35 redhat-uep.pem
-rw-r--r--. 1 root root 2626 Sep  1 10:34 katello-server-ca.pem
-rw-r--r--. 1 root root 2626 Sep  1 10:34 katello-default-ca.pem
[ ~]# 

However when the customer runs the following command he receives a server unreachable error:

[root@ admin]# subscription-manager register --serverurl="<satellite_FQDN>" --org="<org_name>" --activationkey="<activation_key>"
Unable to reach the server at <satellite_FQDN>/subscription
[root@ admin]#