Set an OpenWRT as a Dumb AP while ruing 6in4 tuel?

The computer coect to OpenWRT router through LAN port, the router coect to another router using WAN port.

So it's like: Computer -> OpenWRT -> Exteal Router(Still in private address setup)

What I would like to achieve is disable NAT and firewall for the entire v4 network. (All traffic from the LAN interface of OpenWRT now would be NAT to OpenWRT's WAN address that it got using DHCP) With the involvement of NAT I couldn't initiate coection outside of the router's private subnet.

I tried to follow the documentation on OpenWRT's website about dumb AP. This would be perfect to fix the issue. However, after I finish the setup, I find out the 6in4 tuel that I had on the router couldn't coect anymore. Therefore I couldn't use the tuel on any device coected through the LAN interface.

Is there a solution to this issue? The link to the docs is: https://wiki.openwrt.org/doc/recipes/dumbap