Program started outside of your session knows nothing about X11/Wayland server it needs to connect to nor does it have any of the environment usually set when your session is started. If your user session (and so systemd instance) remains active, you can initiate user service using
systemd-run --user -M bart@.host /home/bart/bin/scripts/radio.sh
It will connect to your systemd user instance and create transient service running the given command in your user environment. See man systemd-run for details.