@Stefan-s As @Douglas-Kryder said, you need to modify your /etc/opkg/distfeeds.conf file and uncomment the following repo:

src/gz reboot_base http://downloads.lede-project.org/snapshots/packages/mipsel_24kc/base

Afterwards, you can issue the following to install the ethtool package:

opkg update opkg install ethtool

And there you have it!