uci show network.wan shows IPv4 only, yet nslookup returned an IPv6 address. Your WAN has no working IPv6 internet route.
Take a backup first then try this:
uci set dhcp.lan.dhcpv6='disabled'
uci set dhcp.lan.ra='disabled'
uci delete network.lan.ip6assign
uci set network.wan6.auto='0'
uci commit
/etc/init.d/dnsmasq restart
/etc/init.d/network restart
Nothing happened, Google still won't open.
Did you clear client cache? switch off/on airplane mode then try again. If that doesn't work, then try forcing custom Google and Cloudflare DNS servers:
uci set network.wan.peerdns='0'
uci delete network.wan.dns
uci add_list network.wan.dns='1.1.1.1'
uci add_list network.wan.dns='8.8.8.8'
uci commit
/etc/init.d/network restart
Tried doing that before when I first got the issue and it didn't fix it.
So apparently setting DNS on my WiFi device to 8.8.8.8 fixed the problem, but it would be a hassle to set them one by one since I have a lot of wireless devices.
Is it still possible to be fix the core issue at the router's side?
As I suspected, your ISP's default DNS is broken or hijacked. If I were you, I'd call & complain to ISP letting them know about broken DNS and missing WAN IPv6
For workaround just set upstream DNS on OpenWrt like I mentioned & force DHCP to hand out 8.8.8.8 to all clients:
uci add_list dhcp.lan.dhcp_option='6,8.8.8.8,1.1.1.1'
uci commit
/etc/init.d/dnsmasq restart
Thanks, the issue is gone now.
The missing IPv6 is my fault, changed one setting that broke it but since I reverted it also, it's back.