It seems you were right, for some reason the newer BIOS was only available from ASUS USA site and not in Finland and the WinFlash was unable to download it. However now it's updated and this bug still exists.
# sudo dmidecode -s bios-version && sudo dmidecode -s bios-release-date X550JX.209 10/31/2015 ** Changed in: linux (Ubuntu) Status: Incomplete => Confirmed -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1528005 Title: rtl8821ae has no connectivity in 5GHz networks To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1528005/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs