Installing current stable (21.02.2) on an AVM FRITZ!Box 7330 SL (with stock firmware) following the guide at https://openwrt.org/toh/avm/fritz.box.wlan.7330 (using LAN1) proceeds as expected, including the concluding reboot.
But after that, the device cannot be reached at 192.168.1.1, neither via browser, ssh nor by ping.
Did you already try an older version of OpenWrt, at least for basic testing?
lantiq SOCs prior to vr9/ VRX2xx are seeing very little real-world testing (as they're very low end by today's expectations and more modern/ cheap VRX2xx devices can also cover ADSL uses), so it's quite possible that no one has noticed more recent breakages.
Can you please give a complete report what works and what does not. I have one and need to determine if I keep or toss it. Thanks a lot. AVM boxes, sadly, are not being lucky past 19.0.