@David Meyers - The upgrade process is not very clear. You have upgraded to the Team Plan which requires a minimum of two paid seats. You will want to reach out to support to request that your account be converted to a Consultant Plan.
As a professor, you may also want to explore applying for the Education Plan. However, keep in mind that it will require you to use your educational institutions email address.
As for still being promoted that you have reached your board limit, do you now have two teams showing in your Miro Dashboard on the left-hand black vertical bar? (Perhaps a new team was created and your Free Plan team is still active?) What could also be happening is that you need to sign out and back into Miro and maybe even clear your browser cache.
Following your advice now - so impressed with your prompt reply - Cheers! dMeyers