When MDM certificates expire, can I still use my iPhone? My dad gave me an old iPhone 7 from his employer. When the MDM certificates expire, will  I still be able to use my phone? 

I asked him to un-enroll my phone, but he couldn’t find it in his system.
I would suggest you do the following things to manage it:

Your dad's company MDM must have synced with Apple Business Manager and as your dad couldn't get it in MDM, then can you ask him to check it in ABM and release the device from there. (assuming your dad has a control on it). https://support.apple.com/en-in/guide/apple-business-manager/asmec4d28461/web
Have you checked whether you can remove the profile or not as you couldn't find the device in the portal? Assuming that the device is still there, in the MDM portal, there must be an option &quot;Allow MDM Profile Removal&quot; which you can check and then manually remove the MDM profile from your iPhone.


I guess these two methods could be ideal at this moment for you.