Ubuntu 18.04 uses now Netplan to configure the network interfaces, so the configuration must be done in the file
/etc/netplan/50-cloud-init.yaml, the documentation advises not to mess anymore with the old file
/etc/network/interfaces. I have used this configuration with my Ubuntu Server virtual machine and it works so far, just make sure the info is correct; the
optional: true setting supposedly speeds up the booting time by not verifying if the interface is connected or not, this is default, also there is no need to declare values not used, for example DHCP, if they are absent they are taken as disabled, also the default renderer in Ubuntu Server is
networkd so there is no need to declare it. Taking the information from your post, it should be like this:
addresses: [192.168.1.1, 188.8.131.52, 184.108.40.206]
Once you save the file, run
sudo netplan --debug apply the
debug flag will output more info and can help to detect any errors. Check the ethernet cable, if in virtual review the VM configuration. If using a WLAN I have read that it is a bit more tricky to setup but I haven't yet set up a machine connected to WiFi with this server version.
If you want more info about Netplan there is a website, it has some basic configuration examples.