Hey @Wenmiao and welcome to the Fido Community!
Before leaving the country did you contact us to cancel your service? If so, the final day billed would be the day you requested to cancel. If you had a balance left on your device then that would go on your final bill.
That said, is it possible we were never contacted to cancel the service? If that's the case then billing would continue as normal even if there is no usage (you'd still be billed for your plan).
I do hope that gives some insight! In terms of your actual bill we're going to look at it together. I'm sending you a PM so we can do that, see you there!