This is a belated answer so hopefully it can be of use to folks who search for and find this question.
After you have eliminated antennae or network locks as the possible culprit, searching issues on the iPhone 6 are usually related to the baseband IC. The baseband IC is very sensitive, just like the Touch IC's on the 6 Plus; it also suffers from poor quality traces and pads.
Go to your phone’s dialing keypad and dial *#06#. You should see the IMEI displayed.
If the IMEI is not displayed, the Baseband IC has to be reballed and two small copper traces (U1/V1) have to be rebuilt and solidified. This is commonly referred to as the “long jumper” solution. You can check out this answer (Service is "Searching" indefinitely) (by @teetopp) as it summarizes the issue really well.