CDMA or GSM?

[AC Question]

I bought an unlocked htc one m9 on ebay advertised as AT&T gsm. After about 3 days, it would no longer connect to the AT&T network...mobile data setting said disconnected. We took it to AT&T. They looked at it and told us it is CDMA. The seller insists it is gsm. Is there any way for me to tell for sure what it is? I want to sell it, but I want to make sure it will work for a potential buyer.

 

[Slenda Mon]

go to settings, about device, (or similar). look for the build of the device. google it and check for yourself if your version of the m9 is CDMA or gsm