By default the ip command used to restart the network interfaces requires sudo permissions.
To allow OctoPrint to manage this for us, we need to update sudoers using the below command:
echo 'pi ALL=NOPASSWD:/sbin/ip' | sudo tee /etc/sudoers.d/octoprint-ip
If your system does not have ip installed, you can install it via apt:
apt-get install iproute2
Install via the bundled Plugin Manager or manually using this URL:
https://github.com/jonfairbanks/OctoPrint-NetworkHealth/archive/master.zip