OK so according to crt.sh | admin.doxat.ba
You have a valid certificate:
ID: 4536300239 Obtained: 2021-05-16 Valid Until: 2021-08-14 for admin.doxat.ba
Logically it should be renewed any time, but will continue to work for you until it expires, or you can get it renewed successfully. (I am guessing that is your intent)
The cert you are serving for "admin"... is valid only for strapi.doxat.ba and doesn't include your "backend" 
I recommend you use/renew that cert via the method you used to initially obtain it.
Somehow I don't think that was the answer you are looking for?
EDIT: AUTO RENEW! I am not familiar with dokku or it's auto renew feature(s) Someone else here will have to pick up to help you resolve this. Sorry for the confusion I understand now.
In the meantime this thread might help?