With your LMP starting on 11 January you'd likely ovulate between 22 and 26 January. It would be VERY early for you to ovulate on the 17th.
So, if you ovulated by the test, then it could be your ex.
If you ovulated by the "average" dates it could be the "other guy."
You'll need a DNA test to determine paternity. Ovulation test kits are not always reliable.
I'd bet on "other guy" but there is a chance it is the ex.
Hope it helps.