Application access & UPnP must be activated on your device:

Home Network > Network > Network Settings > Access Settings in the Home Network
[x] Allow access for applications
[x] Transmit status information over UPnP

Credentials are not needed and, as such, you may want to consider using http instead of https as those requests are significantly faster.

Allowed fields (limited to a max of 4): ["connectionStatus", "uptime", "maxDown", "maxUp", "down", "up", "received", "sent", "externalIPAddress", "externalIPv6Address", "externalIPv6Prefix"].

widget:
  type: fritzbox
  url: http://192.168.178.1