So you have the hostst connecting for a short time (like 1-5 minutes) and after that disconnecting, is that right? You need to be sure that the ESX Servers are able to connect back to the vCenter using 902 TCP/UDP port. As ESXi does not have telnet, you can try using a VM on the same vmnic/subnet as the ESX servers to check this. I would recommend to turn off the vCenter Windows firewall and all other stuff (antivirus, antispyware, etc) that can be messing up with this.
Indeed, service and rpm are not present on ESXi on any versions.
To restart services on ESXi: services.sh restart
To check installed packages: esxupdate query
