# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet static
        address 192.168.10.250
        netmask 255.255.0.0
        dns-nameservers 192.168.10.1
        gateway 192.168.10.1

sudo apt-get remove network-manager
sudo reboot

# network manager cli
$ sudo nmcli d
DEVICE  TYPE      STATE      CONNECTION
eth1    ethernet  connected  Wired connection 1
lo      loopback  unmanaged  --

$ sudo vi /etc/NetworkManager/NetworkManager.conf
[ifupdown]
managed=false
unmanaged-devices=interface-name:eth1  # <--- Add this line

$ sudo service network-manager restart

$ sudo nmcli d
DEVICE  TYPE      STATE      CONNECTION
ens33   ethernet  unmanaged  --           # <--- it's now unmanaged.
lo      loopback  unmanaged  --