‘No Cell Coverage‘ battery usage

Why is ‘no cell coverage’ consistently the highest user of battery life?

While travelling overseas (without roaming enabled) I rely solely on a mobile wifi router with a data-only SIM procured for the country that I am visiting. 

• I have manually disabled both Cellular Data and Bluetooth on my iPhone. 
• Also, I manually enable ‘Low Power Mode’ because my goal is to maximise battery life from a single full charge. 

Necessarily, Wi-Fi is enabled and of course the SSID for my mobile hotspot is specified as the targeted Wi-Fi network. 

• Also, my iPhone is never out of range from that Wi-Fi router.

With the settings described above my iPhone should never be searching for a cellular tower signal and it is never experiencing a low strength Wi-Fi signal from my mobile hotspot. 

Even with all of the carefully considered settings and usage habits described above, the application using the most battery life is invariably reported as: 

‘No Cell Coverage’ !

My goal is to maximise the battery life from a single battery charge and for this reason I manually set low battery mode even when connected to a charger or a power pack. 

Why is it that the ‘No Cell Coverage’  process continues to consume the most battery life of all processes running on my iPhone even when I have deliberately disabled both Cellular Data and Bluetooth?

This makes no sense to me!

I believe I have heeded all advice offered at:

If the battery in your iPhone drains too quickly - Apple Support

The most lucid explanation of (any) phone behaviour in this regard was found at:

"no cell coverage" is draining my battery. - Apple Community