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Hey there,
Unfortunately it sounds like it's possibly a faulty fader or board causing the unit to register activity (hence not entering demo mode - it thinks it's in use).
Was the service centre one of our authorized service centres? They're able to diagnose and resolve most issues as well as reach out to our service team for assistance.
If they are authorized and are still unable to fix it, I wonder if they deemed the repair costs to be worth more than replacement? If it's beyond one year from date of purchase, it's outside of the warranty period, so you may want to inquire about the reason they were unable to fix it for you.