Insurgency: Sandstorm does not support TrackIR/FreeTrack natively (developer-confirmed), so the freetrack protocol will not connect.
What to try instead: the Connect to Any Game guide's alternative routes - virtual joystick (if you can bind camera/lean axes to a joystick) or mouse emulation via opentrack. No confirmed working setup has been reported yet; if you find one, please share it on our Discord.