I wasn't aware that you could use the AT&T model in Europe, I was under the impression that the modem was not compatible, have you tried flashing with the European firmware? Install Odin on your computer, go to here
https://www.sammobile.com/firmwares/galaxy-note4/SM-N910F/BTU/ and dowload the firmware file (takes a while), when downloaded put it in the Odin folder. Turn off your phone, then restart whilst holding the 'Home' 'Volume Down' and 'Power' button at the same time, keep holding until a warning about flashing a ROM can harm your phone appears, release the buttons then use the Volume up button to go to download mode. Open the Odin folder on your computer and start Odin, connect your phone using the USB lead and wait the message 'Added' to appear, select the 'AP' button and go to the file you have downloaded (it will have a .MD5 extension and select it, then hit 'Start' and wait for it load, when finished the phone will reboot a couple of times. Please note you flash this phone at your own risk, while I have never had a problem doing this in many years it does have a small risk of going wrong. It is not as difficult as it appears, and writing it down takes twice as long as doing it. Phil