I tried from the same mobile but either it fails to losd the wallet or it says that Mutiny is already running. I tried on desktop firefox and there is no option to restore from json file.
What can I try next?
Discussion
If it says mutiny is already running, close the app and wait 2 minutes. Also make sure it is not running on another computer.
Firefox should have the import file option. It's in emergency tool in the settings page.