login to fritzbox user interface:
[In case of connection problems:
Use a second network card with a different last octet in the same network segment
#990*15901590* for only reboot.
#991*15901590* for reset to factory default.
#99** for reset of all VoIP settings.
you will need to (temporary) set a fixed IP on your PC in the same subnet (e.g 192.168.178.250)
if your PC has not found a DHCP server, your network card may have been allocated an IP in the 169.254.x.x range