We are getting host not resolvable error randomly in all browser for any site . After 2 or 3 re try or refresh the sites are openning. We are using DNS address shared by our ISP and configured as suggested by MWG documents . Anyone can suggest.
that error message indicates MWG was unable to resolve the IP address correctly. When you did 2 or 3 attempts and the web site then shows up I assume when you do another 2 or 3 attempts it always shows up correctly?
If so MWG was able to obtain the IP and put it into the DNS cache. It is not possible to tell that DNS is broken without further data, but it seems to be the case. I would recommend to switch to a different DNS and see if issue persists or the situation improves. For example you could tell MWG to use Googles public DNS 188.8.131.52 at least for a while to test if the issue goes away.
If the issue persits it is more likely that a networking problem occurs, so I recommend to get in touch with support.