@Richard, Thanks for posting in Q&A.
From your description, I know you want to know what happens if your device offline during the renew days.
Based on my research, if the Intune MDM device certificate expires, the device will stop communicating with Intune, and all sorts of important Intune stuff will stop working, like Intune device configure policies.
Moreover, if you have a device that was offline during the renewal period, you may need to manually unenroll and re-enroll the device into Intune.
Therefore, it is vital that renew the certificate before expiration.
Here is a link about how to renew the certificate.
https://call4cloud.nl/2022/10/fullmetal-certificate-the-revenge-of-renewal/#part1
Non-official, just for reference.
Hope it will help.
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