Hi dd213 - welcome to the forum.
There are a number of ways to use Slot.it oXigen components with Scalextric digital. However, at no point should you be changing the firmware on the ARC Pro powerbase. I suspect you have installed controller firmware onto the powerbase and that is why it no longer functions. Normally, it should be possible to re-instate the correct BLE v2.3 firmware, using the nRf tools app from here: https://www.scalextric.com/uk-en/arc-firmware-update
However, if there is no BlueTooth connection to the powerbase, I suspect you will need to return it to base to be installed via the USB port.
If you are looking to use the ARC Pro system normally, but with better controllers, the Slot.it SCP2 with oXigen module controller is the correct upgrade. It is the SCP2 controller that requires the firmware upgrade to be able to connect to the ARC Pro power base. The ARC Pro power base uses the original Scalextric firmware (BLE v2.3). You will need to use Scalextric Digital compatible chips - either Scalextric C8515 digital plugs, Scalextric C7005/6 retro-fit chips or the Slot.it SP15B SSD chip. The Slot.it oXigen Type C1 chip is also compatible when flashed with SSD firmware and configured to work with AC power - but that’s a complicated and expensive solution to running ’normal’ ARC Pro. Other oXigen chips (A, B and original C type) do not work with ARC Pro or previous Scalextric digital powerbases.
In the future, you could migrate to a full oXigen system - or a hybrid version - using Slot.it controllers with oXigen firmware and oXigen chips. You can use the ARC Pro powerbase to provide track power for oXigen and the Scalextric pit lanes and lane changers are compatible with oXigen. No firmware updates are required for the Powerbase or lane changers. You can use ARC Pro controllers with oXigen - they will need a firmware upgrade from Slot.it to do this. Lap timing and race management in oXigen is achieved by placing magnets under the track and using the oXigen dongle attached to a computer running oXigen race mangement software.
If you have more questions on the controller firmware upgrade, I strongly advise that you contact Slot.it - I am sure they will take the time to respond. If the ARC Pro powerbase does not respond to the correct firmware being re-installed, contact Scalextric Customer Services.