How did our ancestors survive weather conditions worse than 26 degrees outside?

Well, sometimes they didn't.

You have to understand that our ancestors tended not to live in areas with the worst weather conditions for their eras. There's a reason why human civilization advanced quickly among the late Gravettian-era peoples of Europe (i.e., the Solutrians ca 20,000-17,000 BCE): they were living in the relatively temperate areas, like southern France.

The people who did live in areas with extremely cold weather were in the minority. They bundled up in thick animal skins and furs and spent a lot of time indoors by roaring fires.

But while they usually lived long enough to reproduce and ensure the survival of the species, the difficult conditions, among other things, often killed them off at relatively young ages.

When it got this cold...........................they stayed inside and only ventured out to feed and water their animals.

worse than 26 degrees celsius?

by not being too active and not covering up

With animal pelts, inside caves and setting bonfires.

They made fires, shelters, and kept warm bundled up.

Typically, a combination of

- insulated clothing outside

- a method of heating indoor spaces inside

In some cases

- insulated shelter, without heating, with insulated clothing worn inside

Methods of heating the indoor spaces vary wildly

including the rather grotesque method of keeping humans and domesticated animals inside the home all winter long

and spreading the resulting human and animal feces on the floor of the home

the decay of which produces warmth

and (coincidentally and not relevant) raised the level of the floor by several inches by the time warmer weather arrived.

The history of coal mining goes back at least several thousand years.  A coal-burning stove can keep a room very warm.

Apparently they were a lot smarter and more resourceful than we are.  And they certainly were more tough.

uhh these things called fire and shelter

If you go into the ground it's a constant temperature year round so caves would have provided ideal shelters. Once fire came around it helped a lot too