Different subnet cannot resolve IP via Pihole

[Coffee]

OS: Synology
Upstream router: eth1: 192.168.20.0/24, eth2: 192.168.21.0/24
IP of Synology: 192.168.20.4
IP of Pihole: 192.168.20.4 (setup as "Use the same network as Docker Host")

IP within 192.168.20.0/24 is able to resolve IP via Pihole but not another subnet 192.168.21.0/24. I can see the DNS traffic from 192.168.21.0/24 on Synology with tcpdump but there is no return traffic from Pihole.

Does anyone know additional network setting is required for another subnet?

[DL6ER]

Check out your DNS settings on the Pi-hole dashboard. Is this box ticked?

If so, there you have the answer why queries from eth2 aren't replied to. This is typically a good choice, however, in your particular case not. Chose one of the other two settings and it should work.