Ubuntu 18.04 stopped using the classic /etc/init.d/networking and /etc/network/interfaces method of configuring the network and switched to some thing called netplan.

sudo apt update
sudo apt install ifupdown

Example:

source /etc/network/interfaces.d/*

# The loopback network interface
auto lo
iface lo inet loopback
 
allow-hotplug enp0s3
auto enp0s3
iface enp0s3 inet static
  address 192.168.1.133
  netmask 255.255.255.0
  broadcast 192.168.1.255
  gateway 192.168.1.1
  # Only relevant if you make use of RESOLVCONF(8)
  # or similar...
  dns-nameservers 1.1.1.1 1.0.0.1

sudo ifdown --force enp0s3 lo && ifup -a
sudo systemctl unmask networking
sudo systemctl enable networking
sudo systemctl restart networking

sudo systemctl stop systemd-networkd.socket systemd-networkd \
networkd-dispatcher systemd-networkd-wait-online
 
sudo systemctl disable systemd-networkd.socket systemd-networkd \
networkd-dispatcher systemd-networkd-wait-online
 
sudo systemctl mask systemd-networkd.socket systemd-networkd \
networkd-dispatcher systemd-networkd-wait-online
 
sudo apt --assume-yes purge nplan netplan.io

Adjust the values according to your system (network, interface name…).

Because Ubuntu Bionic Beaver (18.04) make use of the DNS stub resolver as provided by SYSTEMD-RESOLVED.SERVICE(8), you SHOULD also add the DNS to contact into the /etc/systemd/resolved.conf file. For instance:

/etc/systemd/resolved.conf

....
DNS=1.1.1.1 1.0.0.1
....

and then restart the systemd-resolved service once done:

sudo systemctl restart systemd-resolved

The DNS entries in the ifupdown INTERFACES(5) file, as shown above, are only relevant if you make use of RESOLVCONF(8) or similar.

https://netplan.io/

https://netplan.io/faq#how-to-go-back-to-ifupdown