A Bit Over-My-Head Creating a Bridge

Apologies up front if this is painfully 100-level, but I'm at a point where I need to toss to folks with more experience.

I'm a mostly-not-network-focused IT / AV / webdev guy working for a high school. Most networking stuff is handled by a dedicated team, but they're a) overstretched and b) partly created the mess I'm trying to fix, so I'm trying to handle this myself.

Here's the scenario: we have an HVAC-type device that has remote-monitoring capability through a wired Ethernet connection. It's in a very inaccessible location with no Ethernet ports. We contacted our Network team, and they installed a wifi repeated instead of an Ethernet jack in the room. The device does NOT have wifi capability.

So, trying to be clever, I am trying to use an OpenWRT-loaded router to bridge the connection. I have the first steps working: the router connects to our WiFi, and if I plug a PC into the router, I have internet access and can ping local static IP devices on the wired network.

What I need is to assign a static IP that another PC on the network can use to reach the HVAC controller. I can't figure out quite how I manage that. The router itself is receiving an IP from DHCP, and I can ping that from a computer on our main network. But I can't communicate to the PC behind the router.

I'm at the point, obviously, where my knowledge of even the terms to search for is running dry. Any help would be appreciated.