This is a bit odd.
The 520 Series SSD should be recognized as a mass storage device (hard drive) on both the SATA 2 and SATA 3 ports without any action on your part.
It goes without saying that the most common cause of this kind of problem is a power or data cable with a poor or intermittent connection, so it never hurts to double or triple check the connections at both ends for both. Use a bright light if necessary to confirm that the connectors are fully seated.
Change the data cable if you have a spare. Most of the SATA data cables I see are so poorly made that I toss them in the recycle after one connection...if I ever have to disconnect the unit to which it is attached I replace the data cable as a matter of course.
You might try dismounting the SSD and see if it will be detected by the BIOS when attached to a SATA 3 port if placed freely in the case. Sometimes the location of the device or the port on the motherboard is located in such a way that the cable connector is strained or distorted when attached and a vital connection is disrupted. This possibility is more likely if the port is not easily accessible or the cable routing requires sharp bends, especially if the bends are near the end connectors.
I'm not familiar with the BIOS setup options for current ASUS motherboards...in the past they have offered extensive choices not present in the BIOS setups for Intel-branded motherboards, so there is the possibility a BIOS setting is affecting the detection/initialization of the SSD when connected to a SATA 3 port. I can't imagine what that option might be or why one would want it, but this is all speculative on my part.
One other possibility: does your motherboard have a dedicated SSD port (other than the SATA ports) where one may connect a small capacity SSD directly to the motherboard and cache (accelerate, in marketing terms) the system hard drive? If so, consult your owners manual to determine if there are limitations associated with that feature that may affect an SSD attached to a SATA 3 ports.