Yesterday I had a new Postpaid data plan for which i have to pay a fixed monthly rental upto relevant range. Which type of Cost it is?

Your Monthly Fixed Charges for Post Paid Data Plan  could be recognised as Monthly Rental and the Charge is considered as Fixed Cost...just like your Monthly Car Rent, House Rent, Memberships etc....even if you use it or not..you have to pay for it.

If you are using the same exclusively for the company purpose and is permissible as per sanction, the claim over such expenses are treated as usual Revenue Expenditure.

If you are employed in a production unit..depends upon the nature of work involved this may regarded as Variable Expenses giving a part allowance to the Fixed nature of the expenses...(cost absorption/allocation). 

THE COST YOU PAY MONTHLY UPTO A SPECIFIC RANGE WILL BE FIXED COST AND THE EXCESS CHARGES WILL BE VERIABLE COST THAT WILL DEPAND  ON USAGE OF DATA.

AND COMPLETE COST EITHER BE DIRECT OR INDIRECT DEPANDS ON THE PURPOSE OF USAGE IN COMPANY.

this type of cost called semi variable cost 

the fixed cost to paid until relevant range is fixed 

the cost that increased after this range and different from month to other is variable 

example of this is telephone expense 

semi variable cost is increase with increasing units and decreasing with decrease in units ( this treatment as total ) 

as per unit semi variable cost decrease with increase in units and increase with decrease in units