From what I can understand, endothermic reactions use thermal energy from the environment to proceed. That means in cold temperature endothermic reactions cannot happen(?).

But can endothermic reactions occur at sub zero temperatures?
What is the lowest temperature an endothermic reaction can happen?

Endothermic reactions can still occur at low temperatures. They will result in the surroundings becoming colder. I don't think there is a lowest temperature at which they can take place.