Your best chance of keeping this alive is to have purchased it from a more reputable supplier - Springhill, Guerneys, Michigan Bulb (I believe they are all related) tend to offer a lot of rather iffy stock, not to mention mislabeled plants. I'd assume the plant was quite small, like in a 4" pot. If that's the case, combined with shipping stress and summer heat, the little plant was likely at risk when you received it to begin with. Rather than planting it out directly at that stage, it might have been prudent to repot and grow on for a while until it recovered from the shipping and developed a sound enough root system to survive inground.

FWIW, most zone 8 locations are pretty much ideal for growing hydrangeas of all types and Royal Red is no different or more difficult to grow than any other bigleaf hydrangea. A location in partial shade (am sun OK), in rich, organic soil and with even, consistant moisture, especially in summer, is all that is required.

As a further note, it may be helpful to know that your "red" hydrangea, should it survive, will not exactly be red......or certainly not the intense red coloring that appeared in the (doctored) photo. At best, "red" flowered bigleaf hydrangeas are a dark pink - sort of a cherry pink coloring - and that coloring and its intensity will be further influenced by siting and soil conditions.