The root of this problem is that there never has been any industry-standard way for an added video card (in a PCIe slot, typically) to send out to the mobo the video card temperature sensor signal. So no mobo maker can be sure that any video card plugged into its slots will give it such info to use for fan control. There is at least one mobo maker who also makes video cards, and they have created their own "brand-specific" non-standard way to do this. So, on some of their mobos IF you also install one of that same maker's video cards, this trick can be done.
Other video card makers have a small part of this done, but not so you could use it. Many video cards come with management software that somehow can display the cards' temperature, and ALSO allows YOU to set up your own version of how the card's cooling fan responds to temperature changes. So obviously they have a way to make their card's temp sensor info available through the PCIe bus, but there's still so standard way that a mobo maker could rely on.
There are some third-party fan control software tools that include the ability to control the video card fan system. I have to presume that they can only work on cards that the software maker has all the secret non-standard suff for and that you must customize the software a bit when you install it.
However, that does offer another route for you to investigate. As I said, many video card makers' card management tools include a way to observe what their own on-card cooling fan control system is doing AND allow you to customize that, just as you can create custom fan cooling curves for a CASE fan controlled by the mobo header. Maybe that is how you can change your card's settings to run its fan faster at high temperatures.
Another possibility is to use that common feature of the mobo SYS_FAN headers that allows you to specify your own custom fan speed control curve, even though it is still based on the temperature sensor on the mobo, and NOT on the video card temp. You could just tell the header that controls fan(s) that blow toward the video card to run faster than the normal pre-defined curve would do. This would "over-cool" the mobo, but would increase air flow in the video card area.
My last possibility would require a bunch more custom work and some preliminary info search first, and might void the video card warranty! Since the card has its own cooling fan and its own fan speed control system based on the card's own temp sensor, you MIGHT be able to tap into the fan power (and speed) control system on the CARD and feed those signals out also to an external standard case fan. That way the fan would be powered and controlled by the video card, not by the mobo. Before doing that you would need to know whether the video card's fan signals are really the same as a standard case fan uses. Then you also need to know the limit of the power the video card can send to its fans. After all, the video card is not designed for any and all standard case fans, and for adding extra fans to its outputs.