If you have the time: hang the print, with small weights attached to the bottom of the sheet. With large prints the weight of the paper is enough to stretch it out.
In higher humidity, like 70% and up, it'll go flat overnight. 60% and lower it might take 1-2 days.
I use a thing called a Wundarail to hold the prints, and then use paper clamps as weights. Spread 3-4 weights along the base of the print, depending on the width of the paper.
I have no idea what the generic term is for a Wundarail, and the local reseller's website seems to have died. It's a horizontal aluminum tube, with a narrow opening down its length, and it's full of glass marbles. Just slot the print into it and the marbles grip the paper. I've got 3m of this in my print room and it can hold 2 meter long, 111cm wide prints without any hassles.
Edit: what I have is basically a 3 meter long restaurant order holder.
Search for images of 'restaurant order holder' and you'll see what I've been talking about.