I'm on Mission 6 and getting hammered hard by the yellow Chinese opponent. My guard houses and towers are manned by under levelled soldiers like privates and my storehouses has piles and piles of coins just collecting dust. The soldiers hasn't been upgraded for over an hour. What gives?

With one coin you can upgrade up to 4 soldiers at once, if they have different grades in a single military building.

I think all the coins you had went to military buildings inside your settlement far from the enemy's border. Coins were wasted for a single soldier. You can click on military buildings far from the enemy, and click on "Evacuate soldiers". Then they will go to storehouses or the HQ, and if their grade is higher then a private they will go out first to enter the military buildings at the front.

There's an option in the Logistics settings, so military buildings far from the enemy border can fill the maximum number of soldiers. Just set all sliders to 100%.

Useful shortcuts:
i - inventory
b - buildings
l - logistics
m - map
s - settings menu
c - statistics

. - switch between military buildings
Shift + . - switch back between military buildings

, - switch between storehouses
Shift + , - switch back

Here's a trick to get the most out of a coin: https://i.imgur.com/HghdSiL.mp4