Hi,
Apologies for the late reply.
By default, AdGuard Home sends DNS queries through the VPN when a tunnel is active. In such cases, if the VPN tunnel cannot reach the upstream DNS servers configured in AdGuard Home, DNS resolution will fail.
Please try SSH-ing into the router and run the following commands to force AdGuard Home to send DNS queries through the WAN only:
sed -i 's/explict_vpn/nonevpn/g' /etc/init.d/adguardhome
/etc/init.d/adguardhome restart
# To restore:
cp -r /rom/etc/init.d/adguardhome /etc/init.d/adguardhome
/etc/init.d/adguardhome restart