The process of how I got it working is documented in the different messages of this post, but I’m gonna do a summary of what I did in case it helps someone:
1.- Go to LuCI (the web interface of OpenWrt, on my Opal router is under More settings -> Advanced)
2.- In the top menu go to Network -> Firewall. Here, in the Zones you’ll have the wan zone restricting traffic to everything behind your Opal router, you need to accept traffic to the lan interface and uncheck the Masquerading and MSS clamping
3.- On your ISP router, go to Routing section (this will be different depending on your model) and add a destination to your Opal router subnet 192.168.8.0. As an example this is how it looks on mine:
And I think that’s all you need, now you can access the devices behind your Opal router from your “main” subnet.