Can someone please give a tip where such an file can be found?
The link in the description does not work and the standard sysupgrade file here makes no difference to the factory image.
Theres an unofficial sysupgrade that you can apply to enable the 2.4GHz radio. You can download it here, specifically the “openwrt-ath79-generic-asus_rt-ac59u-v2-squashfs-sysupgrade.bin” file.
It seems that all threads must be answered by myself. 
The solution is that the link was saved wrong in the Wiki and has been corrected now.
The working link is https://github.com/looi/openwrt/releases
For this router the file is https://github.com/looi/openwrt/releases/download/qcn550x-tag/openwrt-ath79-generic-asus_rt-ac59u-v2-squashfs-sysupgrade.bin
Usually you get "true" name of ASUS router by creating config backup from OEM firmware
eg https://firmware-selector.openwrt.org/?version=24.10.4&target=ramips%2Fmt7621&id=asus_rt-ax54 - all devices would call backup file AX54