I am not sure - but for the infant to use the seat you will most likely need a car seat. I guess they'll let your kid travel as a lap infant without the seat being used

Not sure what the bassinet requirements are but usually it is weight and/or age and 17 mo's might be too old/heavy.

Better check.

QR website is not clear:
https://www.qatarairways.com/tradepo...d-Infants.html

This is what I see on AA. I suspect QR might be similar?

Reserved seat
If your infant will travel in their own seat, you must buy a ticket. The infant must either travel in a safety seat approved by the Federal Aviation Administration (FAA) or be able to sit upright in their seat without assistance and have their seatbelt securely fastened during taxi, takeoff, landing and whenever the 'fasten seatbelt' sign is on.

Bassinets and changing tables
Changing tables are available in the lavatories of all wide-body aircraft.
Bassinets are available on a first come, first serve basis at the gate for travel only on 777-200, 777-300 and 787 aircraft.*
Bassinets are not available in First / Business cabins.
*Infants using bassinets must be younger than 2 years old and weigh no more than 20lbs (9kg)