Not so easy to answer! Blowing on the sim won't help.
Seemingly the antenna in the new phone does not work as well for one reason or another as the old one (which should really not be the case) - nothing else you can specifically do - other than put in a mobile booster antenna in your house to deal with it - not ideal or that cheap (below)
https://www.myamplifiers.com/?gclid=Cj0 ... oLEALw_wcB
So aside from booster option, you may either need to change your mobile service provider so not Orange/HITs? or sell the iphone for another mobile/cheaper Android phone? Perhaps the iphone 6 has been dropped once or several times and the antenna or connection within has been slightly damaged - without pulling the phone apart, guesswork - and too expensive to bother to fix on an older iphone.
I had issue with Orange years back, no matter what mobile we used, was poor reception in the house, gave up in the end and switched provider, problem solved - we couldn't find another solution (other than the mobile booster, but signal was poor locally, not just in-house at the time).