From checking the templates under resources/templates/provision/yealink there is no variable assigned to set syslog.server. You could either try to get this into the fpbx code by submitting a PR, or just add it yourself and populate it through the ui.
Sorry for the late reply, more pressing issues came up that I had to deal with. I will add the setting to the UI to see everything work. If everything goes well I will submit the changes on github.
