Your providers DNS might not be working temporarily.
To use your own, you can change that either in your router directly (it’s web configuration pages)
which will affect all devices connected through that router
or you go through NetworkManager.
which will only affect this one particular computer
In the GUI, you select the 5th tab (IPv4 Settings)
and change “Automatic (DHCP)” to “Automatic (DHCP), Addresses only”
and put your custom nameserver in the DNS Server field below.