Face ID doesn’t use the front facing (selfie camera) but rather the TrueDepth/infrared camera with the dot projector and flood illuminator. If you’re having issues, it’s not because of dust in your selfie camera. Two entirely different systems.
You need to have Apple perform diagnostics on your iPhone. It can be done in person at an Apple Retail Store or at an Apple Authorized Service Provider. You can also contact Apple Support and request remote diagnostics.
If you’re in the US, please call 1-800-MY-APPLE, or outside the US, consult the support articles below. In Canada, call 1-800-263-3394.
Contact Apple for support and service - Apple Support
Genius Bar Reservation and Apple Support Options - Apple