@ChrisPS Have you contacted the Apple group specifically responsible for APNS certificates: https://support.apple.com/en-us/HT208643

You better get on that REAL quick like, if you've only got several days to renew.  I went through a similar experience last year after taking over the Jamf admin role.  I had to provide Apple proof that I was an employee, get a statement from HR...it took a few days to resolve.

Edit: And re-reading - are you sure thats the account that was used to create the cert?  I get mixed up at times with one of my dev accounts and use it to try and sign in to Apple Business Manager or the Cert portal.  Check your account and make sure its the correct one.

My recommendation after you get this taken care of - do your renewals quarterly.  That was a good piece of advice that I got from another Apple admin.  Keeps you on your toes with the renewal process and gives you time to make any changes necessary.

I have no idea what was preventing me being able to sign in, but thankfully this morning I was able to log in again. My certificate expired almost 24 hours ago, but there must be some sort of grace period as I was still able to renew and the devices appear to be working fine with the new certificate. I think I'll be doing the renewals at least every 6 months from now on to give me a bit of time in case something happens again - Apple support is borderline useless in my experience.

I've had the same issue in the past with my account. Was trying to login to a portal but was getting a message saying the account isn't allowed. It then started working the next day. I think it has something to do with accounts that got upgraded from consumer to be part of a managed ID program such as dep/vpp.

That is the way to do it. I do it like 2 months out now, cause I am paranoid lol.