Problems with OpenVPN when using Relayd

Hi all,

I've recently created a setup similar to this one, using Relayd.
My main router is the ISP router (which I didn't touch aside from forwarding the VPN port) and the extender is where I've installed OpenWrt with Relayd.
I was able to get everything to work fine, but I'm struggling with OpenVPN. I'm able to connect to the VPN, but I'm not able to access anything in the network (not even the IP of the extender or the main router).
The best I could do was by adding the VPN to the Relayd interface. In this case, I can access the extender and the devices connected to the extender wifi network, but I can't access the main router and the internet. I also get multiple "IP packet with unknown IP version=15 seen" messages in the system log.

I also tried playing around with the firewall rules and routes, but without success. Needless to say that my knowledge is very basic…
Can someone help me with this?

Thanks in advance!