Guys...I think I've found the last puzzle...
Through all these years, I've only activate iMessage twice.
One time it worked for no reason, but later after few months it stopped to work again.
Another time was that I tried with my real iMac 2011 SN/MLB/UUID, it worked immediately.
But I kinda feel it unsafe for my iMac and improper to use that old SMBIOS for my skylake system.
Anyhow, I've been following this tutorial and many others such as using MAC address as last 12 digits of UUID.
But none of them works.
So I've been wondering, this ain't some difficult guide. Why there are there people reporting working while others come up with totally opposite story.
Until last week, my client sent me an invoice pic through iMessage to my iPhone.
iMessage is not that popular in my country and I was previously a dedicated Android user.
That was the very first time me using iMessage with other people with real Apple device.
Strange thing happened few days after, when I logged in my newly built Hack, iMessage started to work!
I was in disbelieve so I proceeded to log in my XPS 9570 Hack, another success!
What surprised me is that my XPS is not even using a broadcom wireless card.
It is an Intel AX200 patched by AirportItlwm.kext. Everything just suddenly starts to work.
I didn't clear log cache, no Apple ID log out/in.
My SN code was straight randomly created with OC configurator.
I didn't even regenerate them before trying to log in my XPS and I'm sure it failed previously.
So if anyone never success and always comes up with customer service code.
Try log in your Apple ID on a real Apple device then use iMessage or FaceTime service with other device with different ID.(Maybe requires that ID has working iMessage)
I'm guessing maybe what we've been missing is that randomly generated ID needs to be certified by Apple server.
Doing above process is one way.
Real Apple SN codes were already certified in factory so they always work.
And I believe many of Hack users were Android only users like I used to be or just never use iMessage.
Hope my experience helps.