Cellular Is dropping signal constantly

my phone will say I have service, and then 10 minutes later it will say no service although I haven't moved. when I do have service, everything works properly. calling texting etc. I've tried all the recommended steps like resetting network settings, taking out sim, hard reset, airplane mode, restore from iTunes. it must be a hardware issue seeing as I've even set it up as new phone a couple times. How can I fix this?