This is an issue with some browsers and some phones, as the cookies aren't saved and kept as long everywhere, + some security settings. I am afraid I did not make the save system, so I would have no way of knowing how to make an import/export system.
One solution that has worked for some on phone, is switching to another brower or:
One final method is--
- On desktop, start the game.
- Once loaded, right click on the grey BG of the game.
- Choose the one that says somehting like "open in new frame/window".
- This page that is created here, when you do that, it circumvents some of itch stuff that can mess with the saves. So this version of the game might be more robust (depending on what's the issue with your platform/browser/security-setting).
- you can send yourself this link that appears in the address bar if you want to play it on your phone!
Here's the current version of the public game version with this: https://html-classic.itch.zone/html/13013338/index.html