Did you try adding the certificate to the certificates section of the developer portal?
https://docs.mendix.com/developerportal/deploy/certificates/
For on-premise, you can add the certificate yourself.
Try the steps given in https://mkyong.com/webservices/jax-ws/suncertpathbuilderexception-unable-to-find-valid-certification-path-to-requested-target/
Probably it helps already.