So problem appears only when 802.1x radius auth is used with tunnel WLAN and impacts only the remote users, Local user on same site as ZD and radius servers works just fine. correct?
Is the problem same when you don't tunnel the traffic via ZD?
if answers is yes to above then you should start looking AP-ZD MTU and Tunnel MTU setting on ZD GUI and similar ones on your router/firewall/VPN/radius server.
Keep us posted on how it works out.
PS: What does ZD say for Reason for disconnection "after few seconds automatically disconnected from the network"