Hi,
first of all many thanks for your support.
Unfortunately this did not work…
Here is what I did:
In the beginning I updated uboot. I now have the version where you can configure if the button should select the NAND/NOR mode. I enabled that and put the slider into the NAND-position.
I then flashed openwrt-ar300m-3.216-0321-1679391450.img but there is no indication of when the procedure is finished as the device never seems to boot again (no wlan visible either).
So after a while I started uboot again to flash ar300m16-3.216-0321-1679391734.bin.
This time the device rebooted in NOR-mode, I could log in via the web-interface.
However when I now reboot the device it comes up in NOR mode - so flashing the img-file does not seem to have done anything.
What could I try?