GTM Virtual Servers down after translation - Need Help

This looks like a problem with the NAT, are the BIGIP DNS and server behind the firewall ?

BIGIP DNS will monitor the VS which is configured in the IP address field which most of the time is the public ip( the translated ip is the real ip of the server )

Depending on how your network is setup, but if your real server lets say 1.2.3.4 sitting on a dmz is natted to 100.100.100.100 on outside and your DNS is also on DMZ then you need a nat on a firewall for your DNS lets say 5.6.7.8 mapped to 100.100.100.200, by doing this you are literally telling bigip dns to ping 100.100.100.100 from 100.100.100.200 which makes sense.

If you have multiple BIGIP devices then you might think of configuring the prober pools.

Hope this helps !! Thanks, Pankaj