How to curve fencing?
Spend a few dollars on a piece of OSB and build a hot box that will let you steam and bend the 2×4 in the curve you need. Also use a form to set the curve slightly tighter than you need. This will over bend the 2×4 but it will spring back a little. It may take you several tries to get it just as you want it, but it can be done. Typical steam time is 2-4 hours per inch of thickness for soft wood. So the 2×4 will be 4 to 8 hours of steam time depending on the direction you are bending it. Also wear gloves and eye protection, a face shield is suggested, and have a bucket of cold water and some rags to help set your bend. If you use a form, the set is more consistent and can be repeated.
I am assuming that you want to curve the rails and not the styles. The simplest answer is to put the poles closer together so that instead of an actual curve, you have a series of small arcs. If however, you want to actually bend 2×4 rails. you could build a steam tray and soften them, then fasten them in place or build a jig to form them to the desired radius. you must realize that you will be bending them on the flat and not on the edge so the look of the rails may be different then the rest of the fence.
