Your board support X-Fast RAM, so it should work. It sounds like you initially used the version of AXTU from the disk supplied with the board. If so, try the version on your board's download page, rather than from another board. It seems each board has its own version of AXTU.
There's no reason I can see why X-Fast RAM should not work on your board, given the correct version. ASRock has a tutorial about it here:
ASRock XFast RAM
Did you get all the necessary drivers loaded, including the Intel Management Engine Software (IME)? If you check Device Manager, are there any yellow '!' symbols anywhere?