Question about Canon EF 2x III teleconverter

Yes, the converter fits this lens. I myself also use the converter with this lens. However, you have to focus manually. At least with a Canon 5D Mark IV, the "normal" autofocus can no longer be used with this converter. Alternatively, you can use the LiveView to focus or, if you have connected the camera via WiFi, also via the phone or tablet with the appropriate Canon app.