I currently can log in unless im on Wi-Fi. This stsrted about a week or two ago. I'm on a Samsung Galaxy 7. I try to log in when at full bars, on 4g. Getting an error that goes something like: Loading tech tree. Logging in. Can't read application data on failed. Logging in.. Then the loading screen just spins.
I checked for firmware and game updates. Everything was up to date. Restarted the phone. I was then able to log in. Still getting the "cant read application data on failed" message, but it seemed to have pushed past that message.
A lot of players are getting this error. ATA claimed it is an ISP problem and refuse to help the players who cannot connect. I'd recommend you join the Discord server and post in the bugreports channel when you get this error.