"I dumped them to my microSD cards with them certificates.".
When you connect online, and the other user using the same copy you did. - Nintendo simply notice two of the same "unique" certificates online or have checked in from two completely different geographic locations, within a certain amount of time. Similar to how Azure works with MFA logins. which would normally be impossible, they just ban your switch.
You should be using two separate MicroSD cards. One for online, one for offline with emunand, that's why it exists.