How can I access all my (old) photos? I cannot find them.

In iOS Settings > Privacy > Photos > ProCamera as well as in iOS Settings > ProCamera > Photos, you can decide if you wish to grant ProCamera access to all photos in your camera roll, or to selected photos only, or to no photos at all (in that case all photos will be saved to the internal Lightbox).

In case you have decided to only grant limited access to a set of selected photos, you will see a privacy button in the Album Overview. Tap this to quickly access the iOS photo picker to change the selection of photos that ProCamera has access to.