The issue is iPhone 6s are easily bent, especially if you put it in your back pocket and you don't have a hard case. In some cases, the bend is so slight, you don't notice with your naked eye. Even a very slight bend can cause your sim card to lose partial contact. If it's partial, your iPhone won't tell you, it just loses the signal. So here's the solution. Remove the sim card. Open up the iphone. You don't have to disconnect anything. Lift the screen assembly just enough so you can reach the sim card metal plate. Use your thumb fingernail to bend down the metal plate on all 4 sides. Make that sim card slot as tight as you can. Put the iphone back together and give it a try. Good luck.