I have a fully enclosed and secured photon (it is physically inaccessible, terrible design I know). I need to use the particle app to let it into a new network.
Do you have to hold the MODE button down?
I can still see it (shown as offline) in my phone app.
There might be some slim chances if you can get the device online again and flash some new code OTA which adds some Particle.function() to either WiFi.setCredentials() or call WiFi.listen().
You just need to set up a WiFi with the same creds as you already have stored on the Photon and get it to connect to it.
I might be missing something here, but rather than send the router to the Photon can you get someone to configure their mobile phone as a hotspot with identical credentials to your router and then put it close to the Photon?