Hi,
You can try add the following code in Start function:
protected override void Start()
{
m_Keyboard = TouchScreenKeyboard.Open("", TouchScreenKeyboardType.Default, false, false, false);
m_Keyboard.active = false;
}
protected override void LateUpdate()
{
m_Keyboard.active = false;
}

In the meanwhile,try to check Hide Mobile Input option in CustomInputField Script of InputField:

Hope it helps you.