Can't Enrol Devices to MDM via DEP

Hi, I've just finished setting up a Mac Mini running OS Server purely for our companies MDM solution. We're enrolled with Apple for DEP and have around 35 devices at the moment that I can't setup.

So at the moment I have an SSL certificate from GoDaddy which is used to secure the services. I also have a code signing certificate which is not install whatsoever as I can't work out how you do so. <-- not sure if it has anything to do with my issue.

The devices automatically detect that they belong to the company, I go through the setup process but when it comes to "This devices is managed by XXX" Download configuration" I get an error on the next screen failed to download configuration from server. In the log in Apple Configurator I get the following messages:

May 24 04:01:46 iPad profiled[249] <Error>: SecTrustEvaluate [leaf AnchorTrusted]

May 24 04:01:46 iPad Setup[255] <Error>: SecTrustEvaluate [root AnchorTrusted]

May 24 04:01:46 iPad Setup[255] <Notice>: (Error) MC: <MCHTTPRequestor: 0x1282105c0> failed to communicate to the server. Error: NSError:

Desc : The operation couldn’t be completed. (NSURLErrorDomain error -1012.)

Domain : NSURLErrorDomain

Code : -1012

Extra info:

{

NSErrorFailingURLKey = "https://osxserver.frontiermedical.uk/devicemanagement/api/device/dep_mdm_enroll";

NSErrorFailingURLStringKey = "https://osxserver.frontiermedical.uk/devicemanagement/api/device/dep_mdm_enroll";

}

May 24 04:01:46 iPad Setup[255] <Error>: SecTrustEvaluate [root AnchorTrusted]

Every device I've run I get this message. I have my Mac Mini connected straight through to the DMZ and have access to all the required ports and is accessible from the static IP and hostname above from outside world.

I've also tried using Apple Configurator to enrol & setup devices and get the exact same message.

Please help!!! I've come to wits end.