iOS device doesn't connect MDM server

I have a few devices that don‘t connect MDM server.

It work well after enroll the MDM server, but it won't work anymore after a few days.

MDM server send apns request, apns server always return success, and i try power off / on, restore network, restore all settings, but still don't work.

What could cause this situation,and is there a solution?

iOS device doesn't connect MDM server
 
 
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