$ sudo openssl dhparam -out /etc/ssl/dhparam.pem 2048
Point to a trusted certificate chain file. This must contain the intermediate & root certificates (in that order from top to bottom). See exemple_full_chain.pem
Get the root and intermediate certificates from where you buyed the certificates. For me it was: https://www.alphassl.com/support/install-root-certificate.html
For Let’s Encrypt just use the fullchain.pem file in /etc/letsencrypt/live/yourwebsite/fullchain.pem