My iPhone 6 Plus is losing signals.
I'm stunned to know that you have the same problem with your iPhone which I have in my iPhone 5. I'm unable to see signals in my device and the thing is even if I'm at an uncrowded place where my network signals should be full there is no network or hardly 1 or 2 network lines visible in my device. So to solve this issue I tried restarting and rebooting my device many times but it did not work at all.
At last I had to visit Service Center where they said it will be alright and the problem will be solved and the customer support people did a hardware replacement after which few days my device was working properly but later I have same problem with it. So moral of the story is go for a new one, this problem will not solve. Customer support people even said that it might be my Airtel Sim card that is not working well but the same Sim card works perfectly in any other smartphone.