script to change your upstream router DNS to google dns. edit as required for your chosen DNS provider.
#!/bin/sh
# Network Configuration
# Disable peer/ISP DNS
uci set network.wan.peerdns="0"
uci set network.wan6.peerdns="0"
# Configure DNS provider to Google DNS
uci -q delete network.wan.dns
uci add_list network.wan.dns="8.8.8.8"
uci add_list network.wan.dns="8.8.4.4"
# Configure IPv6 DNS provider to Google DNS
uci -q delete network.wan6.dns
uci add_list network.wan6.dns="2001:4860:4860::8888"
uci add_list network.wan6.dns="2001:4860:4860::8844"
# Save and apply
uci commit dhcp
uci commit network
/etc/init.d/network restart
/etc/init.d/dnsmasq restart
/etc/init.d/odhcpd restart