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 188.8.131.52,184.108.40.206;
Add the DNS server IPS separated with a comma [,].