NAT swithes unsuccessfully when dialer interface is down in multi-exit networkin

There are three dialer interface to public network on USG2000 configuration, when the first Dialer2 interface is down, internal network can not access public network. Configuration as follow:
#
interface Vlanif2
pppoe-client dial-bundle-number 2     
#
interface Vlanif3
pppoe-client dial-bundle-number 3
#
interface Vlanif4
pppoe-client dial-bundle-number 4
#
interface Cellular5/0/0
link-protocol ppp
#
interface Ethernet0/0/0
pppoe-client dial-bundle-number 1
#
ip route-static 0.0.0.0 0.0.0.0 Dialer1
ip route-static 0.0.0.0 0.0.0.0 Dialer4
ip route-static 0.0.0.0 0.0.0.0 Dialer2
ip route-static 0.0.0.0 0.0.0.0 Dialer3
#
policy-based-route 2 permit node 1
  if-match acl 2000
  apply output-interface Dialer2
policy-based-route 2 permit node 2
  if-match acl 2000
  apply output-interface Dialer3
policy-based-route 2 permit node 3
  if-match acl 2000
  apply output-interface Dialer4
#
nat-policy interzone trust untrust outbound
policy 0
  action source-nat
  policy source 192.168.1.0 mask 255.255.255.0
  easy-ip Dialer1
#
nat-policy interzone trust untrust1 outbound
policy 0
  action source-nat
  policy source 192.168.1.0 mask 255.255.255.0
  easy-ip Dialer2
#
nat-policy interzone trust untrust2 outbound
policy 0
  action source-nat
  policy source 192.168.1.0 mask 255.255.255.0
  easy-ip Dialer3
#
nat-policy interzone trust untrust3 outbound
policy 0
  action source-nat
  policy source 192.168.1.0 mask 255.255.255.0
  easy-ip Dialer4
Alarm Information
none