iPhone constantly searching for carrier (no service)

First, check the time and date is correct, as this can cause this issue.

Second, check that the IMEI and Modem Firmware is visible in settings. If it is not, you have what's called a baseband issue, and the device needs board level repair. There's a link on my profile for a website of trusted techs if you wish to find someone close to you.

If both of those things are OK it may still be a board level issue (iphone 6 are very common for cellular IC fractures) but it definitely needs to be investigated further by someone experienced