Evening,
Licensing a single host as per the article below will not cause any VM interruption in any way:
I suggestion you do it then try to bring into vcenter again. You may have to go through the logs to find the cause of the error. Since it's a remote site make sure DNS works from that site and you don't have a firewall issue. I have seen what you suggested happen when the firewall was open for vcenter to reach host but not for return traffic. Remember that the host may initiate traffic to vcenter so that path needs to be open too.
Let me know if any of this helps.
Thanks,
J