Installation via EVA: In the first seconds after Power is connected, the bootloader will listen for FTP connections on 192.168.178.1. Firmware can be uploaded like following:
ftp> quote USER adam2 ftp> quote PASS adam2 ftp> binary ftp> debug ftp> passive ftp> quote MEDIA FLSH ftp> put openwrt-sysupgrade.bin mtd1
Note that this procedure might take up to two minutes. You need to powercycle the Device afterwards to boot OpenWrt.
Alternatively you can use this script (Python), to do it for you by following this guide https://fritz-tools.readthedocs.io/en/latest/
AVM provides a recovery tool for Windows: https://download.avm.de/fritzwlan/fritzwlan-repeater-450e/other/recover/