how to ping from PC of VLAN to local IP inside DMZ

Hi all, I'm using a FortiGate 100E with the configuration below:

- VLAN1 (IP 10.123.10.20) go through WAN1

- VLAN 2 (IP 10.123.20.20) and DMZ (default gateway 192.168.100.1) go through WAN2

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I have setup DMZ local IP for a PC is 192.168.100.20 and any PC in VLAN2 can ping to this IP. But I can't ping from any PC of VLAN1 to the IP 192.168.100.20 even when I have set up the route policy of VLAN1 to DMZ as same as the VLAN2 to DMZ. Otherwise, I can still ping from VLAN1 to default gateway 192.168.100.1

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Can you guys please help me with this case? Many thanks.