Before proceeding, power off your OpenWrt 18.06.1 ext4 system and create a backup image.
Upgrade all non-core packages
You know the drill. If a netifd update is available, upgrade it first. Upgrading netifd will restart the network interface daemon, and if netifd is upgraded together with other packages, they will fail to download while the network interfaces are being re-initialized.
root@OpenWrt:~# opkg update root@OpenWrt:~# opkg list-upgradable root@OpenWrt:~# opkg upgrade netifd root@OpenWrt:~# opkg list-upgradable | cut -d ' ' -f 1 | xargs opkg upgrade