I had the same issue.
This is one of the best documents on the internet i found.
Make sure you save the cert after you export it from your push server as a ".p12" file.
Creating the cert with the com.apple.mgmt."name" and saving the exported cer as a ".p12", seemed to do the trick for me.
Then just click and update the "MDM" cert in settings>Certificates with the p12 file.
Hopefully that helps.
I'm gonna try this, thanks
ps. I already used the document you mentioned, it is one of the best.