If you have multiple device using the same Apple ID, you can use the same PhotoRoom subscription on these devices. In that case, you can use the "Already Paid?" button on the new device when you want the subscription to be activated.
To do so, please go to the PRO page. You will find a "Already Paid?" button to restore your purchase at the bottom:
If this doesn't work, we have seen some users succeed by deleting PhotoRoom from the second device (not Pro yet) and reinstalling it and tapping "Already Paid?"