wget -c https://example.com/big.iso-c continues where the partial file ends. The same with curl:
curl -C - -O https://example.com/big.iso-C - tells curl to work out the offset from the file on disk, -O keeps the remote file name. Both only work if the server supports range requests, which almost all do.
For something that keeps dropping, wget -c --tries=0 --retry-connrefused retries until it's done.