Hello Juan
Based on the information you have given us, it is not possible to determine the reason for the problem you are facing. However, if you have determined the problem to be that the AP is indeed in Meraki Management mode, then you must perform the migration procedure as you suggested. There are some requirements in order for this to be successful. First of all, here is some info from the datasheet of the product concerning migration:
Included are step-by-step instructions to achieve the migration.
Secondly, here you can see that if you want to migrate from Meraki to the 9800 WLC (DNA center administration), you must have sufficient licensing for the migration. You will have to check on the Meraki cloud subscription that manages that particular AP, if the appropriate licensing is compliant for a migration. If not, you may need to purchase the appropriate license.
This information should be enough to get you started on the migration process. Let us know how you get along and if you need any further assistance.
I hope this has been helpful!
Laz