I understand the server was affected by a hardware failure, however it has been back online for some time. In case you still can't reach it, you can try
sudo mv /etc/apt/sources.list.d/wolfram.list /etc/apt/sources.list.d/wolfram.disabled
This repository shouldn't be necessary anyway since the wolfram-engine package is hosted at
http://archive.raspberrypi.org/debian/pool/main/w/wolfram-engine/
and the reference to the old repository should be removed automatically once you upgrade to the latest wolfram-engine.
