The "Enable device ringfence" check box should appear under the policy. Device ringfence was introduced in version 3.3.0. If you have been working within the same browser with an earlier version of the AppGate SDP, you might not find the check box connecting to a 3.3.0 server.
This is due to the chrome browser cache.
Please go ahead and delete the chrome browser cache and restart the chrome browser. The check box should then appear under the policies.