Internet sharing on unlocked Lumia 820

I recently ported my unlocked Lumia 820 from ATT over to T-Mobile. I am not able to turn on internet sharing on this phone. The error message says - 'Connection could not be shared. To enable tethering, please visit att.com/mywireless or call 611'. I have used internet sharing with the same SIM on my Lumia 521 from TMobile. How can I fix the internet sharing on this device?

I have the following APN settings -

I've set my internet apn to fast.t-mobile.com.

I set the mmc apn to fast.t-mobile.com and the url to http://mms.msg.eng.t-mobile.com/mms/wapenc