Write to bursts.mansard-0v@icloud.com and you will get a real answer from the person who wrote the app. Most questions so far are below.
Trips can only appear where your photographs carry a location. Some libraries have very few: photos imported from a camera, photos sent to you by other people, and photos taken with Location Services turned off all arrive without coordinates. Settings ▸ Your library tells you what share of your photographs have one, which is the number that decides everything else.
You can add any trip by hand from the timeline, and the app will treat it exactly like one it found itself.
Open the trip and correct it — rename it, change its dates, merge it with its neighbour, split it in two, or say it never happened. Corrections survive every future scan: once you have edited a trip, Periplous will not argue with you about it again.
Anything it is unsure about is not put on your map at all. It waits in the review inbox on the timeline until you say yes or no.
This is usually a phone that was handed on. A partner's photographs sync into the same library, and to a computer they look like yours. Periplous knows the rule — a phone is yours from the day it appears until your next phone appears — and uses it only to settle contradictions, never to throw away somebody else's pictures wholesale. Where it gets one wrong, dismiss the trip.
Colour is worked out after the trips are, from a few thumbnails of each one, and it needs those thumbnails to be on the device. If your library is set to Optimise iPhone Storage and a trip's photographs are still in iCloud, Periplous leaves that trip grey rather than downloading gigabytes of your photographs to decorate a screen. Opening those photos in Photos once is enough to bring them down.
Settings ▸ Your library ▸ Look for new travel. It only reads what has changed, so it is quick. If it says nothing is new and you disagree, tap Look again anyway.
Both devices need to be signed in to the same iCloud account, with iCloud Drive on. Settings will say Not signed in to iCloud if that is the problem. The first sync of a large history takes a while and happens in the background.
Periplous Pro is one purchase — buy it once for $22.99, or yearly for $8.99 after a 7-day free trial — and both unlock the same things. On a new device, Settings ▸ Pro ▸ Restore a purchase. A yearly subscription is managed in your App Store account settings, where it can also be cancelled.
Deleting the app removes its database from the device. To remove the iCloud copy as well, use Settings ▸ Apple Account ▸ iCloud ▸ Manage Account Storage on your device.