The cataclym killed all humans.
Assuming the terraforming is young, and the colony is not populated by amish and not mor ethan a few million people, then the cataclysm destroying the space age technology the humans relied on to survive will have resulted surely in total collapse and extinction. These humans are most likely totally inept to be farmers, they relied on automated harvesters and computers to tell them when to send the plow and seeding machine out. They worked in maintenance or high skill machining or engineering and design. Almost all will work secondary (industry) or tertiary (service) jobs. Almost none of them was working the primary sector: food generation.
Now, the cataclysm hits. It destroys all machinery. Food generation and manufacture breaks down. Nobody can fix the machinery. Millions are killed in the cataclysm and the following societal uproar as food grows sparse. SmallMaybe small pockets of dozens of people survive for a little.
Small pockets that have to relearn farming. ButMany will starve in the years till they arefigured that out. The survivors of those dispersed groups will be too few to keep the genetics intact, and too far apart to effectively form one community. They are at best a few hundred, widely strewn over the planet in tiny settlements of one to three families and possibly unable to ever get into contact with one another. But it takes at least a local minimum viable population to keep one citya society alive for a few generations. You need one stable community of about 500 to 1000 individuals within a somewhat concise area. But your demanded cataclysm throwing them back to the stone age precludes that many survivors in one area, if even allowing so many in total. Your edge condition says, everybody will be dead before they even get to mining.
Self Imposed Regression
It's much more believable if the settlers belonged to a religious or societal group that hated technology in the first place and took the terraformed planet as their holy land, and went almost fully into manual farming but for a handful of tradesmen. The technological regression would have been manmade, but survivable: deliberate destruction of their own high-tech to return to their "roots" as subsistance farmers. Two, three, maybe four, generations later and a strict damnatio memoriae that they came from the stars in place, nobody will remember that they were colonists just a century ago.
The preferred light and heat source will be burning biomass or biomass product, such as wood, charcoal or alcohol, depending on how much tech the humans allowed themselves to keep.