Or wire cutters.
On many models the power led is connected directly to the power supply bus. The switch and 5 GHZ leds come from separate chips and the configuration utilities for those chips may not include LED control.
Thank you very much for your help and your very fast answers! But now I made it on my own by recognizing in this forum that there are new versions available called LEDE.
It is a very simple solution and I don't know if all of you will like it but I want to share with you. After trying the following command
I recognized that the LEDs are really switched off which was never the case with the old image. So I extended the start()-function in /etc/init.d/led and added the the last lines. After a reboot the LEDs are now disabled. Perhaps this can be of some help for anybody.
So you're aware, the "LEDE" versions are from a year or two ago when the OpenWrt project had split. They are old versions. 18.06.x are the current versions of OpenWrt and LEDE both, as the projects have re-combined since then. Anything prior to 18.06 is either unsupported (meaning, most importantly, no security patches), or will become unsupported at the start of the year, as I understand it.
Curious, did you find that all those LEDs are actually present in the filesystem? If so, what version are you running?