Close but this is my link to the support downloads page for the RM21706: Chenbro Micom Co., Ltd.
Weird because the only difference in the URL is "www" instead of "usa". But anyways, the first backplane in the list (6G(SAS2) 6-Port Mini-SAS Backplane) is the one.
Couldn't get my hands on the system yet to try it without the backplane. I'm curious if it will work because the backplane itself does not make any trouble when i switch the SATA controller mode to AHCI (Windows boots, all drives are accessable).
Happy Easter!