This issue has been going on for about a year. Plug the iPhone into the PC (it asks you if you trust this phone; yes) and you can see the photo folder. Start to transfer photos to the PC and it either just stops (for me after 48mb) or I get the error: 'this device is unreachable' or 'A device attached to the system is not functioning'. I could transfer photos when I first got the phone so I can only surmise that there was an update that changed things. The only solution that comes close is to change Settings-> Photos ->Transfer to Mac or PC -> and change this to Keep Originals. This switches the pictures from jpg format to HEIC.
-Assume the basic solutions have been tried. (apple certified iphone cord, updated drivers, power settings on phone).
-iTunes has been retired but if I have to use Apple's awful bloatware I will.
-If you are a mac user who has never seen this do not reply. Generic advice is not helpful.
-An iPhone should be as good as a USB stick or any other phone. It should be able to transfer photos.