There are a few things going on here. I'll summarize.
That "modem" is much more than a modem. It's a router, DHCP server, DNS server, gateway (to the modem) and an L2 bridge (switch). We'll call that network device R0.
The OpenWRT device (we'll call it R1) is also all of those things, with the exception that its gateway route points to R0's private L2/3 domain (192.168.0.0/24). You created 192.168.1.0/24 yourself outside of the scope of R0, and did not configure R0 to route to that subnet. R1 is also configured as a NAT gateway for that subnet, which further obscures it from 0.0. The configuration you've rendered has effectively isolated 1.0 from 0.0, as if 0.0 is a public subnet, and 1.0 is a private subnet. 1.0 can get to resources on 0.0, and can use 0.1 as a gateway to the internet, but 0.0 cannot get to 1.0 without configuring the R1 NAT to do so (via port forwarding or similar). This is referred to as a "double NAT".
This brings up a question: Why is device 2 connected directly to R0? This connectivity makes use of both network domains, and I'm not confident that you actually want two network domains based on your question.
And finally, I would suggest a few things to remedy this:
- Use R1 as a bridge, not as a router/gateway. This would make OpenWRT act as an L2 switch, placing all devices on a single L2/L3 domain (192.168.0.0/24) that the modem/router serves network services for (DHCP, DNS, and routing). This is the simplest solution, and I imagine probably what you're going for here. There would be no 192.168.1.0/24.
- Configure R0 to act as a bridge, not as a router/gateway. This one is trickier, because it involves configuring this modem/router. There is probably a way to do that, though I'm not certain without you providing a model number. This way, the modem would be ONLY a modem and L2 bridge, and would only support a single device connecting to it - R1. R1 would get a public internet-facing address on its gateway NIC (provided via R0), and would be responsible for forming a NAT with that public IP and a private subnet (it could be anything you want, but let's keep it from possibly colliding if R0 gets reset and use 192.168.1.1/24). Device 2 would not connect to R0, but instead to R1. If you want to accomplish some kind of DMZ for device 2, you can do so via OpenWRT in R1.
- Get a different modem. Depending on what kind of fabric you're using, this can be pretty trivial. Don't put up with an unruly device in your network that makes you jump through a bunch of hoops. It sounds like the hardware in it isn't great anyways. This is what I would do, in conjunction with option 2. If it's something like a cable modem, the surfboard models are good. I would avoid combination devices, and get JUST a modem that presents ethernet on a single RJ45 jack.