Put them in an upstream, you will have to move the ssl certs to nginx as nginx terminates ssl at the frontend.
See also https://www.digitalocean.com/community/tutorials/how-to-set-up-nginx-load-balancing-with-ssl-termination
See also https://www.digitalocean.com/community/tutorials/how-to-set-up-nginx-load-balancing-with-ssl-termination