Craig,
An AP will connect to a HM one of 3 or 4 ways. DHCP option, DNS, Aerohive redirector or bootstrap config (that is not removed by a standard factory reset).
If you've tried adding the SN to your connect account GUI and it does gives you an error, the SN is most likely attached to another redirector instance and will need to be cleaned up there. If it worked, either DHCP/DNS is most likely redirecting it to your on-prem box before it makes it's way to the cloud redirector service.
You can manually point it wherever you want (via the CLI). In the Connect platform if you have a little person silhouette in the top right corner click on that and hit About HiveManager and you'll see all the information you need to manually connect the AP. Do note that the APs SN will have to be added otherwise the cloud HM instance won't allow it to join still.
The commands you'll need are:
capwap client server name <server name or IP>
capwap client vhm-name <VHM ID>
I usually stop/start the CAPWAP process at this point:
no capwap client enable
capwap client enable
You can see the status of the connection with the following command:
show capwap client
