No. The GSM section of your phone is only useable when traveling outside the area the primary CDMA carrier. You can't use the phone with AT&T. Also note that the CDMA model isn't compatible with AT&T LTE frequency bands.
Than means that when you travel, you can use a nano-SIM card from a local GSM carrier, for local use charges rather than the exorbitant roaming charges. However, this won't work if you are still in an area served by ntelos, even if you're not on contract with them.