Try and break it down to Loxone vs network vs Shelly vs lightbulb issue.
Can you control it OK via `curl` or pasting the URL into a web browser?
What does it return if you do this?
curl http://<IP_ADDR>/light/0
Generally I find once I have full control of a thing via curl, mapping it into Loxone config is straightforward.
BTW in loxone config you probably need a turn=off command in the "Command for OFF" box too.
Also I'm not familiar with the Dimmer 2, but can you directly set the brightness=100 to turn it on, or do you also need to send a turn=on command first?