First, make sure this really is a Truma problem and not a gas problem. There can be enough residual gas in the lines for your cooktop burners to work with the solenoid valve closed.
If you’re sure your gas is on, then your Truma might be in the “off” position. Find the switch in the picture and check.
If this isn’t the problem, call Truma. They provide excellent technical support over the phone.