I just got a new Apple Watch Series 4, but I'm having some issues pairing to my iPhone (an iPhone 8 with up to date iOS).
When iPhone and Apple Watch are near each other, the pairing procedure starts on the iPhone. I can go through, alternatively, the pairing using the "holding the phone over the animation" or the one with manual code insert. After a few seconds on the following "pairing successful" screen, a popup message appears saying that communication between the devices wasn't stable and that I need to restart.
A couple of times I was able to select wrist and crown position, it started syncing but again failed after a couple of minutes of waiting.
I tried restarting the iPhone and the Apple watch, resetting network connections. This happens with Wi-Fi, Cellular and Bluetooth on, with or without Wi-Fi connected to a network.
One thing that I don't understand is why I can't pair using only cellular connection on? I tried moving outside the building, to stay outside the range of any known Wi-Fi access points. In that case, the pairing can't even start. The iPhone says I need to have a Cellular or Wi-Fi connection on (along with Bluetooth) but it appears that I can't do anything without Wi-Fi. The same happens if I turn Wi-Fi off.
What could be going wrong? How do I pair the Apple Watch to iPhone in such a scenario?