I've done all the options here, as well as a clean wipe and restore from Time Machine, and it's still not working. Apple Support doesn't seem available in NZ - or not to me, anyway. My iMac is 27-inch 2019, and they've never worked on it. I also have a MacBook Pro, 2019 and it works perfectly on that.
Any help (that doesn't involve any of the solutions on the linked page above...) is much appreciated!
It's great to have you in the Apple Support Community. We understand that you haven't been able to use iMessage or FaceTime on your Mac. It's our pleasure to guide you.
Based on the steps you took before in the community, we highly recommend collaborating with Apple Support for additional assistance: Get Support
Have an awesome rest of your day!
Thanks for the reply, sadly that isn't useful :( For me, that page says "Apple support options are currently limited. Thank you for your patience and understanding." and links to the standard support pages. The Mac support directs me to the page of solutions I've already done (e.g. Date and time check, apple id check, reset NVRAM, check third-party VPN etc.)