I have a modem connected to the ISP (running stock fw), an TP-Link Archer C50 running openwrt connected to the main modem via WIFI and configured as a simple extender using relayd. This works great.
What I'm trying to achieve is connecting the openwrt router as an openvpn client and as a result, tunnel all traffic coming from client connected to it via ethernet. I followed this guide in order to establish a client connection: https://github.com/StreisandEffect/streisand/wiki/Setting-an-OpenWrt-Based-Router-as-OpenVPN-Client and I suppose that client connection itself works (according the the status and system logs), though the connected clients are not passed through the tunnel. I suppose there's some additional configuration I'll need to do in order to associate the relayd interface with the openvpn interface, though I'm not really how. Can anyone point me in the right direction?
I think what you propose isn't going to work when your openwrt router is running relay configuration and you also want openvpn client on same device. There was a similar post from another member on this forum a few weeks ago. Devices which want to use the vpn tunnel must be on a different subnet for success. https://forum.openwrt.org/t/multiple-problems-with-openvpn/49041
You can check your openvpn client router is functioning in normal router mode when ethernet wired to your main router. See my PDF for helpful advice. https://openwrt.ebilan.co.uk/viewtopic.php?f=7&t=279
Maximum openvpn speed using C50 v3 or v4 is 13 Mbps over ethernet btw.