How to fix iPhone 6 no service?

So I just got an iPhone 6, one of those used ones they sort of "fix" to make it like new, and I have had it for a few weeks, and it's bern fine. But ever since this morning, it will either say searching or no service. Plus I can't use my cellular data. It won't even show who my carrier is. I updated to iOS 11.1 and 11.1.1 and my phone started workout nvm properly for a few hours, but now it is back to searching or no service. I have hard restarted my phone, reset all settings, updated my phone, and removed and put the SIM card back in. Nothing is Working. Can someone help?

Added (1). I want serious answers please

Just break it and go get samsung.

Take it back to the retailer and ask them to fix it or exchange it.

You should still be well within the phone guarantee period.