Edit the GELF Input you are using - all the CERTs settings should be in there.
Also - you are noting "https://<graylogserverIP>:9000/api" … I mentioned it in the end of my last post - most certificates are set up with FQDN rather than IP… something to check.
This post has a lot of the configurations you would be using though his issue is the private key… comparing your settings to his may help.