I bought a new iPhone - iPhone X.
And I can't sign in to my iPhone account on this device.
I get this message: connect the device to the internet
But, I already had to connect it to WLAN and Mobile Data.
I checked the date and time, I checked the connections, I rebooted the phone but nothing from the instructions in support helped me.
What should I do? 
Try first to optimize the range of the mobile network, see this Apple article:
If you see No Service or Searching on your iPhone or iPad
If you see No Service or Searching on your iPhone or iPad - Apple Support
Charge the battery > 70%.
Check Wi-Fi, see the following Apple articles:
Connect to Wi-Fi on your iPhone, iPad, or iPod touch
Connect to Wi-Fi on your iPhone, iPad, or iPod touch - Apple Support
If your iPhone, iPad, or iPod touch won’t connect to a Wi-Fi network
If your iPhone, iPad, or iPod touch won't connect to a Wi-Fi network - Apple Support
If they are On, turn Off Hotspot & VPN.
enter in iphone account
