Greetings Maldon... This machine has been working great with High Sierra.. not very well with 10.14 and newer. I've never tried "iMessage" and for some reason, it's not working. "User/Password" is incorrect-could not sign in. I've confirmed that ethernet is EN0/Built in. I've also tried your iMessage fixer. I've used both iMac15,1 and Macmini7,1 with no success. iCloud/iTunes/Appstore is all good. My NVRam is functioning also. Any thought? Many thanks sir.
Lenovo ThinkCentre M83
i3-4130
HD4400
High Sierra 10.13.6