Can I cancel my plan?

Hello.

I am a AT&T customer with a GoPhone which has a 60$ plan every month. 

I will be stayin in Ecuador until August for vacations and I need to cancel my plan.

I have tried everything to cancel my plan but the only way is to register on the GoPhone page and I can´t since the page sends you a text message with the pin number and I am outside the US to recieve any text messages.

Is there any way I can cancel my GoPhone plan? I have tried anything and the next payment is due the 28th 

Thank you!