[stumped]

i suspect this is due to the new device running fire OS 6, and Op needs to find , on the device, a connect as... menu, which appears only when device is connected, probably as a pull down from top of screen thing
there are 2 ways for device to connect to PC and device is defaulting to the wrong one.
this is all explained in much more detail in other threads
it is a bug/feature of android 6/ fire OS 6 that devices connect by default as media devices, not as storage devices, hence the no storage..
the older device , runnign os 5 , or os 4, would not have this "feature"
android smart phones also require use of this connect as... menu , every time they are plugged in to transfer files

a much simpler solution , for tablets, is to get calibre companion and move the books wirelessly, not via USB, that bypasses the problem
as an example, from a quick google search, here are instructions for a chromebook. the Fire tablet shoudl have a similar menu... you have to locate the option marked in bold and use it
Unlock your Android device.
With a USB cable, connect your device to your Chromebook.
Unlock your device.
On your device, tap the "Charging this device via USB" notification.
Under "Use USB for," select File Transfer.
On your Chromebook, the Files app opens. Use it to drag files.
When you're done, unplug the USB cable.