No internet access - Change of rate plan needed ?

During a US trip 3 weeks ago I activated my gophone account however I did not yet fill it up at that time. 

Now, I added 45 USD with a (Walmart) refill and go (4G LTE) card. A confirmation was received on the phone and I also see that my account balance is 45 USD. However, my account info says "60 USD monthly plan". probably I have entered this during activation. 

Now I cannot access internet on my smartphone. What do I need to do ? Change the monthly plan - how ? Add another 15 USD, which means I can never use the new "USD45 Monthly Unlimited" just purchased at Walmart ?

Awaiting support.

Thanks

lumia-abroad