We are clear and there is no question that Bluetooth Scanner
is pretending to be a Bluetooth keyboard, so iOS thinks that it does not need to show the on screen one.
For Barcode Scanner type socket mobile
are now adding a feature to open the iPad onscreen keyboard. For example, the Socket CHS has a new button press option. You can check if your device have the same functionality*(It is likely to be of some help)*
Bring Up the Virtual Keyboard With a Real Keyboard can be a good idea.
When you have a Bluetooth keyboard connected to your iPad, the virtual keyboard will cease to appear. (This is a good thing.) However, what if, for some random reason, you needed that virtual keyboard? Don't unpair your Bluetooth. Just... Hit the eject key on Apple's physical keyboard. It'll bring up the virtual one.
There is another way to try to solve this, personally I tried it and it was very unstable. Taking into account that it is a kind of hack, expected unstable behavior.
This answer to a related question (https://mcmap.net/q/211593/-show-iphone-soft-keyboard-even-though-a-hardware-keyboard-is-connected) has what seems like a hack based on a UITextField's inputAccessoryView.
Even the user @Kurt Arnlund
in this answer mentions a external app called KeysPlease
link here. I really have not tried it, you can try.
In summary, there seems to be a clean way to solve this, I think these may be the best attempts to reach a solution
Sources:
Bluetooth Scanner sets system-wide keyboard, UITextFields wont show soft keyboard on responder
Force on screen keyboard to show when bluetooth keyboard connected
Show iPhone soft keyboard even though a hardware keyboard is connected