You indeed need to merge your old Mendix account to your current one. In your current account, go to your Advanced settings and enter the email address of your old account. If you still have access to your old account, you can accept the merge there. If you don't, then Mendix Support will review the merge request, which normally takes about 2 weeks to complete.
P.S. Not only your certifications will be merged, but also your earned points, achievements, and Academy progress is automatically transferred. Also, please keep in mind that it is not possible to separate accounts after they have been merged.
Hope this helps!
You can merge two Mendix accounts into one. Go to your profile, click on the wheel (on the right of the page), then goto Advanced. Here you can send a merge request.
Hope that works for you!