Apple Watch Series 3 with Cellular not receiving phone calls
Based purely on what you’ve described, your Apple Watch is working as intended. Let me explain…

When an iPhone is in Aeroplane Mode, its Bluetooth is not necessarily switched off. This means your iPhone and Apple Watch are still in fact connected via Bluetooth and that’s why calls aren’t coming through on your Apple Watch. 

The reason it’s been designed this way, is the assumption that if your Apple Watch is still connected to your iPhone, then your iPhone is in the same area as you (e.g. at home, at work, etc) and that if you’re not wanting to receive phone calls on your iPhone, then you’d also not want to receive them on your Apple Watch. 

To test my assumption, you can either repeat your test but manually ensure that Bluetooth is also switched off on your iPhone. Likewise, you could just go for a walk (it doesn’t have to be very far - 50 metres is enough) so that your Apple Watch loses connection to your iPhone and arrange for someone to call your cell phone.

If I’m right your Apple Watch will ring when your cell number is called. However, if it still doesn’t work, let me know and we’ll investigate this further.