Thanks for the tips @anon82920800 and @vcunat. As both of you implied, I should turn my focus away from Turris (and there, I four the fix). Turns out, that I was able to access my cable modem using a Laptop of a friend. After a bit of comparison:
It was my freedome VPN client (forgot all about it). It does not send calls to 192.168.100.x to to my network.
Thanks for the help! For now, I need to disable the vpn client to access the modem. All good.