Most likely there are networking issues between HCI and HCP pertaining to SNMP protocols, eg firewall blocking SNMP port.
To test SNMP traffic, you can perform an snmpwalk from the command line on the HCI node:
snmpwalk -v1 -c public hcp-node-ip .1.3.6.1.4.1.116.5.46.7.1.1.0
If SNMP is configured properly, you should get back HCP's version
$ snmpwalk -v1 -c public 172.20.23.63 .1.3.6.1.4.1.116.5.46.7.1.1.0 SNMPv2-SMI::enterprises.116.5.46.7.1.1.0 = STRING: "7.3.3.18"
Important note: You need to use the IP of an HCP node as reported by DNS. So first perform an nslookup on admin.cluster-name, then use one of the IPs reported by DNS for the snmpwalk command above.