iPhones do not mount like a hard drive. You are meant to manage their files in the iTunes software. One can use software called iExplorer on Windows to access the "iDevice" like a hard drive.
Not correct. You do get read only access to the camera roll, and that is what the OP's question is trying to regain access to, and that appears in Windows Explorer as the iPhones device name with an internal base folder called "Internal Storage" (then "DCIM" etc.). That is all you get acces to, and as I say it is read only.
If unlocking the phone before connecting it did not help, then check the device drivers. - you say that you've already fone that and found an "unknown device" that you associate with the phoe. Does that come and go with connecting/disconnecting the iPhone?
You whould see two drivers whern the phone is connected.
"Apple Mobile Device USB Driver" under "Universal Serial Bus controllers"
and "Apple iPhone" under "Portable Devices"
You say that the iPhone is seen in iTunes. Does that work reliably? (Device backup runs through successfully on connection?). iTunes 12.5. Had driver installation issues, and the first driver above ("Apple Mobile Device USB Driver" under "Universal Serial Bus controllers") did not properly install on many systems. If it did not, you can manually update the driver by pointing to the correct driver on disk. If you attached a LOCKED IOS 10 handeset to your computer, this is the only driver you should see,. If it does nto appear (or if you see an "Apple iPhone" driver) update the it by manually specifying the location the driver files on disk. The drivers are found at "C:\Program Files\Common Files\Apple\Mobile Device Support\Drivers". If this driver is not properlly installed, then iTunes might appear to see the phone, but it will be uneliable.
If the "Apple Mobile Device USB Driver" does appear, then then the iPhone is connected WHILST UNLOCKED, the second driver - The "Apple iPhone" driver should appear. If you see the unknown device pop up when you conenct the phone, I suppose you could manually update that driver and specify the "Apple iPhone" driver. It is a Microsoft driver so you done need to manually specify a location. If the unknown device is there even when the phone is not conencted, then is it NOT relateed to this issue. Don't touch it.
I'm assuming that you've already tried conencting the phone to a different USB port, are using an original Apple undamaged cable, are conecting the cable directly to the computer (not through a USB hub) etc?