cat /etc/network/interfaces

returned

# ifupdown has been replaced by netplan(5) on this system.  See
# /etc/netplan for current configuration.
# To re-enable ifupdown on this system, you can run:
#    sudo apt install ifupdown

I wanted my network connections to be controlled by Network Manager, not Netplan. If I had to install ifupdown then I wanted ifupdown2.

I tried

apt install ifupdown

Which returned

Reading package lists... Done
Building dependency tree       
Reading state information... Done
You might want to run 'apt --fix-broken install' to correct these.
The following packages have unmet dependencies:
ifupdown2 : Conflicts: ifupdown
E: Unmet dependencies. Try 'apt --fix-broken install' with no packages (or 
specify a solution).

So I did

apt --fix-broken install

Which returned

Reading package lists... Done
Building dependency tree       
Reading state information... Done
Correcting dependencies... Done
The following packages were automatically installed and are no longer 
required:
rsplib-docs spl-dkms
Use 'sudo apt autoremove' to remove them.
The following packages will be REMOVED:
pppoeconf
0 upgraded, 0 newly installed, 1 to remove and 10 not upgraded.
1 not fully installed or removed.
After this operation, 135 kB disk space will be freed.
Do you want to continue? [Y/n] y
(Reading database ... 193626 files and directories currently installed.)
Removing pppoeconf (1.21ubuntu1) ...
Processing triggers for man-db (2.8.5-2) ...

Then I did

dpkg -i ../ifupdown2_1.2.1_all.deb

which returned

(Reading database ... 193619 files and directories currently installed.)
Preparing to unpack ../ifupdown2_1.2.1_all.deb ...
Unpacking ifupdown2 (1.2.1) over (1.2.1) ...
Setting up ifupdown2 (1.2.1) ...
ifupdown2.postinst: Warning: No 'iface lo' definition found in 
/etc/network/interfaces
ifupdown2.postinst: Warning: No 'auto lo' statement found in 
/etc/network/interfaces
Processing triggers for man-db (2.8.5-2) ...

It seems to have installed

apt list --installed | grep -i ifupdown2

WARNING: apt does not have a stable CLI interface. Use with caution in 
scripts.

ifupdown2/now 1.2.1 all [installed,upgradable to: 1.2.5-1]

I then made the following addition to /etc/network/interfaces

auto lo
iface lo inet loopback 

auto enp11s0
iface enp11s0 inet dhcp

added

[ifupdown]
managed=false

to /etc/NetworkManager/NetworkManager.conf and did

systemctl stop systemd-networkd
systemctl disable systemd-networkd
service NetworkManager restart

Everything seems ok but if anyone knows how to upgrade ifupdown2 from 1.2.1 to 1.2.5-1 that would be good.