C# - OnclientClick And OnClick Is Not Working At The Same Time?

I have a button like the following,

<asp:Button ID="pagerLeftButton" runat="server" OnClientClick="disable(this)" onclick="pager_Left_Click" Text="<" />

When I use my button like that, onclick is not firing. When I remove OnClientClick, then onclick is firing.

What I need to do is, disable the button during the postback and enable it after the postback ends.

Edit: Additional information:

I added break point to my firing functions is c# part and I am debugging, they are not firing for sure. Those functions are like

protected void pager_Left_Click(object sender, EventArgs e)
{
//Do smthing.
}
protected void pager_Right_Click(object sender, EventArgs e)
{
//Do smthing.
}

and when I click my button, it is disabled for 1-2 seconds and automatically enabled, but I am not sure why it is enabled. I didn't add any part for it to be enabled again.