We have exactly the same issue that was posted in this article
APNS portal not working | Apple Developer Forums https://developer.apple.com/forums/thread/698419
We downloaded the csr from our MDM to do the renewal of the cert, log into the apple push cert renewal portal, click to renew the cert. Choose the csr downloaded earlier but then the upload button just doesn't do anything at all. Page is not loading or showing any errors at all. Tried different browsers, different computers with the same result on all of them the page just doesn't seem to do anything.