@burnout426 Response from avm.de
https://fritz.com/service/wissensdatenbank/dok/FRITZ-Box-7530/3495_Internetbrowser-meldet-nicht-sichere-Verbindung-zur-FRITZ-Box?t_id=6967464

The Internet browser flags connections to websites that are not encrypted or only partially encrypted as unsafe, as data entered on such websites could be stolen by third parties. However, since you are accessing the user interface of your FRITZ!Box within your home network and no data is being sent over the Internet, you can ignore the message.