Hello, my name is Mark and I will try help you with this.
If you are 100% sure the battery is fine then either the starter motor, the connections to it, the engine earth strap or the ignition switch is faulty.
I suspect your fault is with the battery though even though you might think it is fine.
Have you done a capacitance test on the battery with a suitable tester or have you just switched the ignition on and the dash lights look bright?
Do you still need assistance?