I'm on a raspberry pi 4b
/home/pi# uname -r 5.4.79-v7l+
and trying to set
/sbin/ethtool -s eth0 speed 100 duplex full
The simple reason for that is because it does not work with autoneg / full speed.
Years ago I would have set that in /etc/network/interfaces but from what I googled the last hours that is depreciated. Additionally my pi would not if I do that. To my mind crontab is not a great solution; additionally I want to set eth0 as the primary connection - needed at boot. Where can put that setting ideally?