TMG ISP Redundancy and DNS

Hello,

I have installed TMG with 3 NICs such as ISP1, ISP2 and Internal

I configured the ISP1 and ISP2 interfaces with IP addresses and default gateways and

configured internal NIC with IP address, but Default Gateway.

I installed DNS service on TMG and configured the forwarders pointing to ISP DNS servers.

Finally Internal NIC DNS configuration

Primary : 127.0.0.1

Alternative: Internal AD DNS servers

Configured persistent routes

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Persistent Routes:
  Network Address          Netmask             Gateway Address  Metric
         10.0.0.0               255.0.0.0                      10.1.2.1            1                 ( Internal LAN)
          1.1.1.1            255.255.255.255             192.168.5.1       2                   ( ISP1 DNS Server)
           2.2.2.2           255.255.255.255             192.168.4.2       3                    ( ISP2 DNS Server)
           0.0.0.0                 0.0.0.0                       192.168.4.2    Default
           0.0.0.0                 0.0.0.0                       192.168.5.1  Default

Now I am trying to join the TMG server to domain but failed. Error saying that cannot resolve domain name

I would highly appreciate any help.

Thanks