Yes, you can bring an ordinary camping chair on to the Tønder Festival site.
Here are the conditions: chairs must not be placed right in front of the stage, and may not be used at venues with seating.

Tønder Festival is responsible for everyone’s safety. If we consider your chair is in the way, you will asked to move. Our request will be friendly but firm, and must be followed. A refusal to comply can lead to expulsion from the Tønder Festival site and confiscation of your wristband.