Question about Panasonic KX-TG6811FXB Telephone (j)

The KX-TG6811FXB is a DECT phone, which means that it connects to a base station via DECT technology and not directly via Wi-Fi.
It is therefore not compatible with a direct connection to the Internet-Box 4 via Wi-Fi, but it can be used with the Internet-Box 4 telephone socket if the latter has an RJ11 telephone port.
The Wi-Fi connection mentioned in the product properties seems to be an error or a confusion, as this model only works with DECT.
For optimum use, you need to connect the base of the phone to the telephone socket on the Internet-Box 4.
If you want to use a cordless phone via Wi-Fi, you need a VoIP or SIP compatible model, which is not the case here.
In short, this phone works with the Internet-Box 4 via the telephone socket, not via Wi-Fi.