I can't activate my device keeps on saying searching

so I recently bought an unlocked iPhone 5s from student computers and when I first put the SIM card it activated and I had service but then it all of a Sudden switched to saying searching instead I tried resetting the network settings, rebooting, turning airplane mode off nd on, taking the SIM card out nd putting it back in however nothing worked so I decided to factory reset my phone but now I can't activate it because it keeps on saying searching I even changed the SIM card and it keeps on saying the activation server is temporarily down. I don't know what's wrong with it do u think it would help getting the battery replaced?