Check in device manager if your igpu is actually disabled.
Try deleting the .ini or .xml files that store your game settings, preferences and launching hardware/ software parameters.
Upon launching the game, all deletd files should be recreated anew, hopefully resolving your issue.
Does the game launcher have any specific option that would fix this?
There are only a few settings before launching the game, such as resolution, rendering quality etc. nothing that would indicate setting a primary GPU to use.