I have EERO for the Wifi but the router part is a box running OpnSense.
The wifi is 192.168.7.x and the ethernet is 192.168.0.x
From the ethernet the EERO has a static IP I assigned it 192.168.0.95
Not really sure how to get the EEROs devices onto the same subnet.
Ah....OK, I have a Google WiFi mesh (5 pucks) for the house and ran into the same issue. The mesh has to be converted to a wired Ethernet at some point, so that mesh puck is the gateway for the mesh subnet and runs a NAT while all the other mesh pucks are just Layer 2 devices. all on the same WiFi subnet. Once you cross the NAT, it's a different subnet.
So thinking cap on...what to do?! I also have some Ethernet runs in the house. Internet comes into a basement space where I have a rack with routers, PoE switch, firewall and servers. Basement ceiling is a drop ceiling, so it's fairly easy to run wire to any wall and then pull wire to the needed room. I ended up doing wired runs several years ago to all the key media installations on each floor -- painful but better than WiFi for streaming media.
I use one of these Ethernet runs to my office where my WiiM Amp is located. I run power over Ethernet (PoE) to my office using a Ubiquiti PoE switch, but a single PoE-injector is OK too. I bought a couple of Ubiquiti U6-IWs [1] for my media installations. PoE powers the U6-IW to provide a 4-port wired switch and a new WiFi 2.4/5GHz. I use wired connection from the U6-IW to my WiiM Amp and use the new WiFi SSID from the U6-IW to connect while in the office and it's on the same subnet as the Amp. So, from my wired router, it's a common subnet all the way through to my phone which makes accessing media server (Plex, Emby, whatever) easy since DLNA is on the same subnet as the WiiM and my phone.
So, the end result looks like this: (edited to fix the lines)
ISP router -- my wired router -- L2 firewall -- switch -- Google WiFi Mesh (x5) -- family stuff
'............................................|
'........................................PoE switch -- wired run -- UI-6W -- wired run -- WiiM Amp
'............................................|........................|
'........................................media server.................-- UI6W WiFi -- my phone
Problem solved -- for me and I'm thrilled with it. It was expensive and needs a second WiFi, but it left my current mesh net in place for all its goodness and solved my WiiM control issue.
It may not be ideal for your case. You've come up with a good workaround for getting both WiiM and desktop on the same subnet, but it does have drawbacks in bandwidth. Since you mentioned having some mix of wired and WiFi, my setup may offer some suggestions for your case. Hope that is a decent explanation and helps
[1]
https://ui.com/ca/en/wifi/in-wall