I have seen reports of similar issue a few times and since you have confirmed the same problem shows up on different devices and different platform, I tend to agree this is an account problem.

I have no experience with Xiaomi devices but on the iPhone, could you try to following please to see if you can make it to work?

Turn off internet data (both WiFi and mobile data)
Start the PVZ2 App
Try entering the Adventure mode

If you can get into the adventure game mode, try to play a level and then turn on internet data, try to watch an ads either for free gems or free coins, exit to main menu. Click the cloud button and register your email if you have not done so before.

If you cannot get into the adventure mode, you will need to contact the help center for further assistance. Please note you will be ask to provide your player ID. This can be obtain on your iPhone's system settings menu.

Settings > PvZ 2 > PLAYER INFO