Custom email template send x days after user is approved

Hi,

I would like to add a custom email template that is triggered only after x days after user is approved.

I was able to add a custom email template following these instructions:
https://docs.ultimatemember.com/article/1339-customize-email-templates

Now I am struggling with how to trigger this notification X days after user approval.
I tried to add
wp_schedule_single_event( time() + DAYS_IN_SECONDS, ‘um_after_user_is_approved’ );
but no success.

Anyone would be kind to point me in the right direction?

Many thanks