I have come to the same problem, and as far is I know everywhere I search about this matter ends up on that TP-Link has done the mesh setup on their own way (not over the standards of the industry)
So there is pretty much no chance to replicate the OneMesh setup on OpenWrt so the range extender thinks that it still talks to a TP-Link original setup router.
Some people has cut some corners and they installed OpenWrt firmware on their range extenders so they can setup a mesh network with OpenWrt ways, unfortunately this isn't my case because since I have two RE205 v3.0 that there is no firmware support for them on OpenWrt. Also I have tried with some old TL-WA855RE v2 since there is a firmware of OpenWrt but for the v1 version but without any luck yet.