I posted on servethehome, too, and learned some stuff from those guys.
I've done a lot of port configuration attempts with the webui and CLI. It says that the ports are capable of "2.5, 5 or 10Gbps", and 10Gbps 4x is QDR, but then none of the ports negotiate to more than 5 4x = 20 Gbps. I attempted to force the port speed (both the switch and the hca port) using ibportstate on the node, but trying to force the switch port resulted in an error and trying to force the hca then forcing a renegotiate (pull cable, reinsert cable) caused the link to never come up again until after a reboot.
It turns out that there are DDR only versions of the IS50XX switches. https://cw.infinibandta.org/files/showcase_product/101213.173410.167.IS5030_35.pdf I didn't think there were because ALL of the IS50XX switches are advertised as QDR, including the one I bought. Mine is an IBM rebrand, so unfortunately doesn't have the complete mellanox part number anywhere, but if yours has a "D" in the complete part number, then it's a DDR switch. "Q" means QDR. I'm not sure if this applies to me or not yet.