Have you tried installing/activating it using Native Access 1 rather than the latest version? This article might help - I Cannot Find GUITAR RIG 5 in Native Access – Native Instruments (native-instruments.com). It'll depend a lot on whether the older version of Native Access will run on your Mac, you can download it from here - Native Access Legacy Installers for Older Operating Systems – Native Instruments (native-instruments.com)
Note that a Guitar Rig 7 license won't automatically grant you licenses for older Guitar Rig versions. You would need to buy a license for a certain version second-hand if you don't own one.
As for the Demo/Service Center issue, check this article:
@Deeken Service Center is discontinued. Do you have a M1 mac? Then read this article: GUITAR RIG 5, KONTAKT and SUPER 8 R2 Open as Demo Versions on Apple Silicon / M-Systems
And if anything here helped, please get in touch with my colleagues here: https://bit.ly/NI_support_NA