If you can't access Fritzbox anymore there is hardcoded IP Adresss in the Fritzbox (7590/7530):
You can reach the user interface through either http://fritz.box or the FRITZ!Box's IP address (set to http://192.168.178.1 by default). The FRITZ!Box is also continuously accessible at the emergency IP address http://169.254.1.1. This information was obtained from AVM.
So connect your PC/Laptop to the Fritzbox with a ethernet cable , disconnect the internet from Fritbox.
Set your Laptop IP Adress on the ethernet Interface to 169.254.1.2. Now you can access the Fritzbox with http://169.254.1.1