Certificates are not trusted on Chrome and Safari on old iMac with El Capitan

I don´t understand how these things work but in this post Certificated not Trusted Chrome / Safari. Chain issues incomplete - #7 by Claes the say " Change cert.pem to fullchain.pem". I don´t know if thats my problem or where I can check if it is linking to the cert.pem or fullchain.pem file on my server?