Check the /etc/network/interfaces file. If there is nothing but lo – you are hijacked by the network-manager.
Edit /etc/network/interfaces like:
auto enp1s0f0 iface enp1s0f0 inet static address 192.168.100.120 netmask 255.255.0.0 gateway 192.168.100.1 dns-nameservers 192.168.100.2 184.108.40.206 dns-search MYDOMAIN.ORG
# systemctl stop network-manager # systemctl disable network-manager
replace the symlink /etc/resolv.conf with a proper file.