Hmm, okay, so I can drive over to Canada, make a developer friend there, build an instance of the Signal iOS app using the licenses there, load it onto my phone via TestFlight or USB stick, then drive back to the U.S. and use it assuming TSA doesn't touch my phone?
After you load TestFlight and Signal build onto your phone, make a full encrypted local backup via iTunes.[0] Upload that backup image somewhere. Turn off Find My (iPhone) to disable activation lock. Restore iPhone to factory setttings. Return iPhone to factory sealed box. Optional: mail phone to self at destination or other location of your choosing in destination. Cross border. When at desired use location, unbox phone. Fetch backup you made earlier. Restore backup to iPhone. Use Signal.
Another tip is that it doesn’t have to be the same phone as far as the backup and restore is concerned. Enrollment of the TestFlight app might be impacted if the phone changes but that’s just my concern because I haven’t tested that part.
Here’s some links related to these ideas which may be relevant to your interests.
