dkim on two VPS on same domain

Hi, my main domain is configured and confirmed to be signed by DKIM.
But i tried on my second VPS which is subdomain of same domain. I logged in using the subdomain like sub1.domain.com and edited the exim.conf and directadmin.conf as stated in directadmin tutorial and generated the key.
Is there a different way of adding DKIM to second VPS that is based on same domain with the first VPS?

I tried sending an email from the second VPS but it comes in spam folder (and there is not signed by… information on GMAIL)

thanks