Please note - showmount functionality is disabled by default. It can be enabled with the following command:
cluster ::> nfs server modify -vserver <vserver> -showmount <enabled/disabled>
Also, I am guessing you are new to NetApp ONTAP concepts, therefore I suggest take a look around on google, directly on netapp kb/supprot site or numerous blogs, there is lots of useful & simple information around 'How NetApp/ONTAPP' works, and how data is presented to external clients. Once the concepts are clear, rest will be easy to troubleshoot.
For NFS client to mount, it must be presented with vserver Data LIF, that's the only way you can read/write to NetApp data SVM from remote clients. [Note: SVM root vol is only used as junction-path for SVM to traverse across mount-points,. In short, SVM root volume is the entry point to the namespace hierarchy].
In your command, I see svm root vol, you should not store user data in the root volume of an SVM.
Questions to ask: 1) Have you created 'data volume' in your SVM? 2) Have you created 'data LIF' in your SVM? 3) Is NFS 'data LIF' routable to Clinet (NFS-172.16.28.135 <-->Client-10.24.78.111)