Dell has not validated any video cards for the R710. Only since the 12th generation servers have they validated some videocards (and the server may have power cables to power the videocard).
This means that unless someone else posts a success story with exact details of which card they used, you may have to just try some different cards.
Note that your server has no power cables for the videocard and the PCIe slots probably don't provide more than 25W in power, so you want to be sure to shop for videocards that don't require more than 25W in total power.