Read the [official doc](https://help.ubuntu.com/community/WakeOnLan).
The config doesn't persist. To make it persist after each reboot, you have to edit configuration file `/etc/network/interfaces`.
However, latest ubuntu doesn't have this file. Instead you have to edit `/etc/netplan/01-network-manager-all.yaml`.
For example,
```yaml
# Let NetworkManager manage all devices on this system
network:
version: 2
renderer: NetworkManager
ethernets:
enp6s0:
match:
macaddress: 12:34:56:78:90:12
dhcp4: true
wakeonlan: true
```