You need to contact the visa office,but i doubt they will give you more time.It really depends on the office itself,as some offices ask for it because time is running out for your medical and they need to process it.Others ask for it right away.And if you cant get in contact with them like most times people cant,you need to send it by the timeline they gave you.
I dont want to get harsh or anything but you knew this was coming and this PR means more to you than anything so,i dont get why you question when you should send it and if its because you book a trip or vacation around your PR app then thats the chances you take but it is very unlikely they will extend it.And like the letter or message you got stated.Like all do it says if you do not send it in by the date they have gave you they will assume you want to withdrawl,or they will proceed with the info they got,and since they cant proceed you know what the outcome will be.