I don't have access to this device anymore, sorry.
For future reference, I've tested OpenWrt on this device as a wifi relay using relayd and while it works the wifi is unreliable and unstable.
My sonos couldn't play a song without the loading failing, sometime even the wifi network would disappear temporarily or completely until a hard reboot.
I had to go back to the stock firmware even though it doesn't have all the latest security fixes but at least it works.
Thanks @exadeci for the info and sorry that things were unstable. Did you try filing any bug reports and/or inquiring for help on the forums? Maybe someone will have a solution for you.
I have personally not tried relayd on this device, but I have used wireless to simultaneously connect to a WAN and create a LAN (therefore in standard routed mode), and have found it to be stable (especially with the addition of TravelMate for managing the upstream WWAN).
I'm using the V3 version with it's matching firmware, maybe it had to do with some configuration issues but it seemed to be working decently it was just unstable with relayd.
While I've been back for 24h on the stock firmware and everything stayed connected and I didn't see any issue yet.
My setup was sharing a client wifi to the ethernet and a master wifi and the master would often stop broadcasting.
Here's my config if anyone wants to take a look: https://send.tresorit.com/a#m8fBuO-5-DRGinlUzMM_5A
It's awesome that OpenWrt supports so many devices but for use case it doesn't seems to fit.
Closing this topic now, since support for TL-WR902AC v3 has been added quite a while ago.
Please open new topics for any further questions regarding this device.