Canon ring shower

I bought these tomme tippee insulated portable bottle warmer but not sure how to use it? - portable bottle cooler

Basically, I think, put some warm milk and a warm bottle of these isolates. Obviously, stay warm for 4 hours.
Yesterday, I saw, and after 3 hours, temprerature room for milk, but a little cold.
Today, I have to boil the milk, if you stay longer in the bag. After 4 hours, it was hot, but the nipple and the bottle of milk smelled funny.
My question is: Over-heating of milk, so cool, it's dangerous for the child?
It is the only way I found the bottle to keep warm, up to 4 clock.
To do any other way, so that the milk is warm for long?
Why is this Mothercare bottle warmers are highly recommended if does'nt work