We recently setup the Manage Engine MDM in our on-prem Endpoint Central. iPhones we purchase from Verizon automatically get put in the Apple Business Manager, which is set to automatically sync to the MDM server. I have had one successful user get through the iPhone setup because they were on company Wi-fi. 

I have a remote user trying to finish the enrollment process as a part of the iPhone setup process, and they get an error message "Failed to load web page, Unable to resolve host: <servername>." I confirmed with my network administrator that the inbound ports 8022, 8027, 8443, 8383, and outbound port 443 should be open and the server has a public ip address, but the user still can't finish the enrollment.