It seems to be very specific to LTE (4G+) 1800Mhz. It is nice to read in this forum that it is not specfic to France. In France, only Bouygues Telecom delivers 18000Mhz and uniquely. Other Iphone6s connected to an Orange or Free sim card are working fine according to many discussions since a month now (http://forums.macg.co/threads/perte-localisation-gps-iphone-6s-plus.1271312/page -16) because they don't use the band 3 1800Mhz. These French operators will deliver 1800Mhz next January!
Without any clear explanation neither from Bouygues (http://forum.bouyguestelecom.fr/questions/968426-iphone-6s-perte-gps-b-you) nor from Apple, I returned my Iphone 6s to the Apple Store where I bought it (you have 15 days to return it in Paris). After having my money back, they said that Apple, as it did in the past for similar problem, will call back the wrong Iphone 6s if it is a hardware problem. Nice to hear that. I hope that it would be solved by software for those who can't get it back.
If you want to know if you are connected with 1800MHz 4G, you can, for France, see a very detailed map from the Agence national des fréquences and a new app name cartoradio (https://itunes.apple.com/fr/app/cartoradio-mobile/id694622086?mt=8) or on the web at the URL http://www.antennesmobiles.fr/index.php?geo&show
I don't know if such very precise maps are available (and authorized) in other countries but we would appreciate to know for other countries where 1800Mh is available like Italy for example.