Is there any other way to install firmware than recovery mode or flash tool?
Spreadtrum devices don't have recoveries, nor don't they have a ''download mode". Their bootloader is accessed with a "boot key" (usually hardware buttons) through SCI USB2Serial port.

To get the phone detected, just simply switch it off. Hold volume down button (or volume up) and plug the device to the computer. Windows should install drivers (Download and install SCI drivers). 

Open the flash tool (ResearchDownload), load the PAC file, click on run. Power off the device, hold volume up/ down, connect USB cable. Et voilà.