A motherboard failure under a year is a clear warranty case.
Instead of using chat, look for:
Surface
Device service and repair
Request a call back
Visit:
https://vsa.services.microsoft.com/en-us/v1.0/
Choose: If you’d like to speak to a human agent, let me know at any time.
Tweet @MicrosoftHelps with a short description of your issue
Their social support team responds quickly and can escalate warranty cases — they have real humans monitoring it