Cantera runs entirely on your device, so most problems are settings on the device itself. Here are the ones people hit.
Email soporte@linkr.lat and you'll get a reply. Telling us your iOS version and what you were doing saves a round trip.
Open the share sheet in any app, scroll the app row to the end and tap More. Cantera is in that list — turn it on, and drag it up if you want it near the front. iOS hides new extensions until you do this once.
Reading an image happens once, shortly after it lands, and it needs the app to be open. Open Cantera and give it a moment. Settings shows an Images read count — if it's going up, it's working. Handwriting and very low-contrast text are genuinely hard and may not be read.
Open Settings → Restore purchases. Make sure you're signed into the same Apple Account that made the purchase. Cantera asks Apple directly each launch, so once Apple confirms it, it stays.
Both devices need to be signed into the same Apple Account with iCloud Drive on, and both need Cantera Pro. Sync starts on the next launch after you turn it on — quit the app and reopen it. If a device has no iCloud account, Cantera keeps working locally instead of failing.
Files app → iCloud Drive → Cantera, with a folder per project and an Inbox folder for what isn't filed. It's written as you go, so your Mac sees the same files. If the folder is missing, iCloud Drive is likely off in iOS Settings.
Open the note and change it. The sorting is a small model running on your device, and it will be wrong sometimes — correcting it costs one tap, and the note is yours from then on.
Everything exports as plain Markdown from Settings, at any time, whether or not you have Pro. Nothing about Cantera is designed to keep you in it.
Delete the app and it's gone from the device. If you had iCloud sync on, also remove Cantera's data in iOS Settings → your name → iCloud → Manage Account Storage.