Just wondering if
A) your cellular carrier supports "WiFi Calling"?
B) you have enabled WiFi calling on your iPhone?
C) you have enabled WiFi calling on your Apple Watch?
The Apple Watch always uses the iPhone's radios when it can see the iPhone. It saves power on the Apple Watch, as the cellular radio can be rather power hungry, especially if in a weak cellular signal area.
Putting your iPhone in Airplane mode, generally also puts the Apple Watch into Airplane mode.
If you take the Apple Watch out of Airplane mode, and the iPhone is still in Airplane mode, then if in WiFi range or Bluetooth range of the iPhone, the Apple Watch is going to defer to the iPhone with its larger battery, and better cellular antennas.
Those might be some of the reasons the Apple Watch is not switching over to WiFi calling or using its own cellular radio.
NOTE: It is not really expected that the iPhone will be put into Airplane mode when not on an Airplane, and thus also expect that the Apple Watch would also need to be in Airplane mode. Granted users find all kinds of interesting ways to use features that were not as the designers intended, and as a result find flaws in the implementation. This maybe your situation.