DNS entries stored in
/etc/resolv.confare overriden with every boot.
There are ways to prevent NetworkManager from overriding the file, altering dhcp hooks, or manually editing /etc/network/interfaces.
A simpler solution would be to add the following line to your
prepend domain-name-servers 18.104.22.168,22.214.171.124;
Add the DNS server IPS separated with a comma [,].