That’s why LightBurn thinks the machine is 400 x 350 mm. That’s what the firmware settings say it is.
For homing and movement within the workspace:
The machine doesn’t have homing switches, which means that the zero point of the machine is random, in fact, wherever you have the head when you power the machine will be zero. To set the zero point properly, either power up the machine with the head at the front-left, or jog there and enter this command:
G92 X0 Y0
The above tells the controller “where you are right now is X0 Y0”.