If your certificate was obtained from a major certificate authority (e.g. GoDaddy.com), its setup can be verified with publicly available tools.
If someone else generated the certificate for you via an internal certificate authority the first step is to verify that it's working for systems other than Mattermost, whether 256 or 512.
Please see these troubleshooting steps and confirm with whoever provided the certificate that it can be successfully applied to another system and verified?