OpenWrt AP with OpenVPN split tunnel

I recently moved and now I have to use my ISP router as main router. Unfortunately, it does not support OpenVPN. Therefore, I want to use my OpenWRT router as an AP and OpenVPN client. So far, I got it working that the OpenWRT splits its traffic, i.e. everything that should go through the tunnel goes through the tunnel, the rest goes through the main router - this I could check from ssh. However, the clients attached to my OpenWRT router send all traffic through the main router.
I'm guessing the problem is that the two routers are connected via LAN, because I want to use the ports of both routers.

Is it possible to have the OpenWRT router act as a dumb AP, but still send traffic through the OpenVPN tunnel?

One way I see is to set static routes in my main router to route vpn traffic through my AP, but this is not ideal.. I would have to set up some 30 routes which may change in the future anyways.

TL;DR: Is the following possible:
Main router -> Lan -> OpenWRT router with OpenVPN client -> Clients
OpenWRT router should send client traffic on some routes through the tunnel, the rest through main router.

OpenWrt should either not be dumbAP, or it should be the acting dhcp server in the lan.