Phone carrier problem (iPhone6)

[TreesMadeOfTanks]

So I bought an iPhone 6 off eBay and made sure it was an AT&T phone, I even paid to have the IMEI checked. Out of the box it connects to wifi and works perfectly, in the data section it displayed ''No Service'' so i took the phone to my local AT&T store and went through the entire process of activating it and then the ''No Service'' changed to ''Searching'' the AT&T employee restarted the phone like 5 times and it still said ''Searching'' keep in mind I live in a large metropolitan area and I know that I have coverage everywhere around where I live, the employee told me that the only thing he could think of is that my antenna was broken, so I left the store without paying for anything. Here's where the weird part comes, about a week after i left the store the battery on the phone died, i plugged it up and when the phone came back on it showed full bars AT&T LTE, i could make phone calls and use cellular data, it worked for about two days and now its back to ''Searching''. Should i replace the antenna? i read other places it could be the sim card so i don't wanna fix things that aren't broken. Thanks for any help