Could you rephrase your issue's description a bit? It's not clear which certificate is rejected by what. Please keep in mind we usually don't assist with configuration of 3rd-party software under Community Support, only with issues with our own products. If it's just some 3rd-party Wordpress plugin rejecting Gluu Server's certificate, you should seek help on its dedicated resources.
By default, Gluu Server will use self-signed certificate. If this is what causes the issue, you either need to acquire one properly-signed by trusted CA instead, or somehow add your self-signed cert to trust store of your plugin (if it offers such option).