Also wrong. It is a common mistake mixing DNS servers. It doesn't work as you would expect and it tend to corrupt your internal DNS cache (also on the router). You should use primary and secondary from the same service.
In other words if you use Google, set
- primary 188.8.131.52
- secondary 184.108.40.206
If you use Cloudflare
- primary 220.127.116.11
- secondary 18.104.22.168
If you use mixed IPv4/IPv6 connection you should also fillup IPv6 DNS to match IPv4, or disable IPv6.
If you change DNS servers on the router, restart the router and all connected devices to propagate changes.