This is my experience!!
I use dual server mode(Internal and External EMM), EMM ver 9.55 / OS: Win2008 R2
When my portal certificate was expired, I logon to EMM console
and double click "portal" to change certificate.
After that, some user say EMM not work anyway~
I check certificate was new and not expire, so I found this document
On the external EMM, first import the new certificate,
then I follow the document and bind the new certificate to the website,
so it's work!!
You can use browser to link https://publicdomainname/emmportal
if it cann't display or only display when you use "HTTP" not https.
Maybe you can try my solution, may it can help you to solve!!