Ubuntu 17.10+ disable netplan

These directions have been tested also to Ubuntu 18.04.1 and will very likely work also for any future release using netplan and systemd.

There's no need at all to fiddle with GRUB nor any manual file removal. The configuration set up in /etc/networking files and directories will survive reboots.

These are the verified steps:

1. Check the actual interface names you are interested in with ip l for the links (aka interfaces) and with ip a for addresses.
2. Install ifupdown with sudo apt -y install ifupdown.
3. Purge netplan with sudo apt -y purge netplan.io.
4. Configure /etc/network/interfaces and/or /etc/network/interfaces.d accordingly to your needs (man 5 interfaces can be of some help with examples).
5. Restart the networking service with sudo systemctl restart networking; systemctl status networking or sudo /etc/init.d/networking restart; /etc/init.d/networking status. The output of the status command should mention active as its status.
6. The command ip a will show whether the expected network configuration has been applied.
7. Optionally, manually purge the remants of the netplan configuration files with sudo rm -vfr /usr/share/netplan /etc/netplan.

No reboot is needed in order to "refresh" the IP configuration: it will be active as of step no.5 . In case of troubles, double check the interface names. A typical IPv4 DHCP configuration will resemble this one:

auto enp0s3
iface enp0s3 inet dhcp

while a static IPv4 address can be configured like this:

auto enp0s3
iface enp0s3 inet static
address 192.168.255.42/24
gateway 192.168.255.254
#dns-nameservers 8.8.8.8 208.67.222.222

Beware, the dns-nameservers entry won't work (thanks @Velkan for pointing it out!): the resolver is still using /etc/resolv.conf and systemd is providing his own resolution service from 127.0.0.53. So you can manually update it (no networking restart needed!):

nameserver 8.8.8.8
nameserver 208.67.222.222

But his would be only a temporary solution to vanish after the next reboot.

To get a permanent solution you need to edit /etc/systemd/resolved.conf and add a line like this one to the "[Resolve]" stanza:

DNS=8.8.8.8 208.67.222.222

Please, refer to man 5 resolved.conf for the full documentation.

Finally, in the unlikely case any network service is not responding as expected, then that services may need a restart. But that's a weird non-standard network daemon behavior.

The exact method to do this is hard, perhaps impossible to locate now in the early days of netplan.

I currently have this set to:

GRUB_CMDLINE_LINUX="ipv6.disable=1"

I assume that you mean that your /etc/default/grub reads, in part:

# If you change this file, run 'update-grub' afterwards to update
# /boot/grub/grub.cfg.
# For full documentation of the options in this file, see:
#   info -f grub -n 'Simple configuration'

GRUB_DEFAULT=0
GRUB_HIDDEN_TIMEOUT=0
GRUB_HIDDEN_TIMEOUT_QUIET=true
GRUB_TIMEOUT=10
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX="ipv6.disable=1"
<snip>

I also assume that the link you give suggests that you add the referenced wording:

# If you change this file, run 'update-grub' afterwards to update
# /boot/grub/grub.cfg.
# For full documentation of the options in this file, see:
#   info -f grub -n 'Simple configuration'

GRUB_DEFAULT=0
GRUB_HIDDEN_TIMEOUT=0
GRUB_HIDDEN_TIMEOUT_QUIET=true
GRUB_TIMEOUT=10
GRUB_DISTRIBUTOR=`lsb_release -i -s 2> /dev/null || echo Debian`
GRUB_CMDLINE_LINUX_DEFAULT="quiet splash"
GRUB_CMDLINE_LINUX="ipv6.disable=1 netcfg/do_not_use_netplan=true"
<snip>

I suggest that you do just that, followed by:

sudo update-grub

You will also need:

sudo apt install ifupdown

It may already be installed.

Finally, fill in /etc/network/interfaces manually to configure your network the way you want it.

The exact process to do this post-installation, as far as Google and I can find, doesn't exist. Reboot with your fingers crossed!