If your /etc/resolv.conf gets overridden by NetworkManager on reboot, then you need to use nmcli to set nameservers in the way below:
1 – find the connection you want to edit
nmcli connection show
2 – add the nameservers
nmcli c modify “connection name” +ipv4.dns “8.8.8.8 9.9.9.9”
3 – reboot
Enjoy