No need to delete the Default Gateway entry in NetBalancer, it is disabled after all.
Leave a ping running (ping -t http://www.google.com) and then try to browse. Check for instability in the ping round trip time, as well as packet loss. I hope that you are testing with a wired connection to the ZS and NOT via wireless.
As I said before try another router in the place of ZS temporarily, or if you don’t have any connect your PC directly on the modem and set it up to connect to your internet provider.