Hmmm... I attempted to revert to stock with these instructions https://openwrt.org/docs/guide-user/troubleshooting/vendor_specific_rescue#rescue_firmware_partition but I somehow can't access the 'mini-web server'
edit: turns out that the web server is located at 192.168.0.254 instead of 192.168.0.1
Unfortunately uploading a stock firmware failed at 40% (I will create a new topic for this - see EAP615-Wall reverting from OpenWrt to OEM fails)
edit Jan 13th: I finally succeeded in reverting to stock and will resume tests tomorrow.