Just to clarify:
- You connect your phone to your PC via USB.
- Then on your phone you can see a notification about your phone charging via USB and to tap on this notification to change the connection mode (perhaps you have to swipe down from the top to see it).
- You tap there and change the connection mode to MTP.
- And then your computer still doesn’t show the phone as it should do.
In that case you could try the following at least with Windows 10, I don’t know about Windows XP nowadays …