I guess there is an outdated information about configuration via /etc/default/knxd, that is no longer true. Instead use /etc/knxd.conf to setup the daemon. This is because systemd is used as init system.
knxd is configured as a minimal service by default, so at least you should set -DTRS to enable Server for Tunnel (T), Router (S), Discovery Service (D) and enabling Routing (R) itself.
Furthermore, you have to add your Interface to link knxd to your knx bus. That is possible via many different devices (usb, rs232, ip tunnel…).
Please also ensure to use only unused individual addresses (-e and -E parameter)
Please be aware that you don’t need knxd if you already have a knx/IP gateway and you must not use -S in combination with ip:, as only one Routing interface is allowed, either knxd client or server side.