@admon would you be able to point me to the correct resource?
It worked (Somewhat).
After unchecking dns for vpn and dns for vpn guest, AdGuard Home shows the client IP address instead of localhost 
I then go to VPN Dashboard of GL-iNet (/#/vpndashboard), updated the Proxy Mode from Global Proxy to Based on Client Device. Checked the VPN on the device that is on the exclude list, it can successfully do DNS look ups and bypasses VPN no problem. However, ALL dns requests to AdGuard Home now show as localhost. Even after I revert the Proxy Mode to Global Proxy doesn't help.
I then navigate to /cgi-bin/luci/admin/network/firewall/forwards, changed nothing but clicked Save & Apply and Voilà! Client IP now show up in AdGuard Home again...