Z-Push in zimbra

I try to configure an iphone with my zimbra server (which works well in the browser) Release 8.8.7.GA.1964.UBUNTU16.64 UBUNTU16_64 FOSS edition. On the web the only tool to do it is z-push I follow the tutorials but none works, the closest is the one of link but I get the following error:

  

HTTP ERROR 502   Problem accessing ZCS upstream server. Can not connect to the ZCS upstream server. Connection is refused.

Any suggestions for configuring z-push in zimbra? I try to configure it on the same server using z-push 2.0.8 and the zimbra 56 backend, although I already try the new versions 2.4 and 67 respectively.