Why can't my Galaxy S4 get online even though the SIM card is in place?

I bought the S4 used and otherwise it functions properly. Also bought the Micro SIM card from T-Mobile and activated it (note: my line is under a friend's account so he handled the activation part). It fits and the phone recognizes it. The 4G/LTE symbol even shows up in the status bar but I can't get online, send texts, make calls, etc.

The only other possibility is the fact that my line still works on my old SIM card in my LG G2X with no network issues, and my phone number comes up as "unknown" on the S4 when I go to "About Phone" then "Status" under "Settings". Any help would be appreciated!