S5 won't allow me to add a new apn in order to restore my data access

[AC Question]

Purchased an unlocked At&t Samsung Galaxy S5 and was using through Straight Talk. I accidentally hit restore all defaults option under my Access Point Names menu and now it won't allow me to add a new apn in order to restore my data access, when I hit the plus sign nothing happens (no menu comes up at all.) I was able to add a new apn a month ago when I bought the phone and wanted to set it up to work through Straight Talk. Can someone tell me what to do?