"EAP650-Outdoor" Support on OpenWRT?

I was thinking of buying an EAP650-Outdoor, I found nothing on support for OpenWRT.

How would I go by adding/investigating this?

TP-Link has made the GPL code available on the support page for the device. A quick glance at the source code shows the SoC is IPQ5018. It also runs Linux 4.4, so basically using Qualcomm's SDK.

Unfortunately as of this writing there's basically no support for IPQ5018 in OpenWrt. I would skip this device for OpenWrt purposes unless you want to work on bringing OpenWrt support to the SoC itself (not a trivial undertaking, to put it mildly).