No communication between wired and wireless devices

Situation as follows: I have a TP-Link TL-WR841N running Chaos Calmer 15.05.1, which provides a wired connection to multiple PCs, and a WiFi network (let's call it Network_A for the sake of this post). The WAN port is connected to a Technicolor TG789vn router/modem/whatever hybrid supplied by the ISP, which has its own WiFi network going (Network_B). The TP-Link sits at 192.168.1.2, and the Technicolor at 192.168.1.254.

The wired PC's can ping each other without problem, and so can the WiFi connected ones. However, a wired PC can not ping a wirelessly connected one, or vice versa.
Interestingly enough, those connected by WiFi show their default gateway to be 192.168.1.254, even when connected to Network_A. However, they can not access the Technicolor's web GUI unless connected directly to Network_B. This, along with the fact that changing the TP-Link's IP to 192.168.2.2 induced a change in the IPs of wired PCs but not WiFi devices, leads me to believe that the IPs for WiFi devices are still assigned by the Technicolor.

Now, how could I get both wired and wireless devices to act as if they were on the same LAN (they're not, right? I have very little networking experience)? My aims are merely file sharing and LAN gaming. Ideally, I'd also like this to include the devices connected to Network_B, but that is not strictly necessary. Would I be correct to assume that a VLAN could work in this scenario? In that case, how would I set it up?

ip -4 a; ip -4 r; ip -4 ru
uci show network; uci show firewall

It sounds to me like you should just turn off the wifi on the Technicolor device. Then all your devices will be connected to the TP-Link OpenWrt device and be able to talk to each other.

Also, your device is ancient and has only 8MB flash and 32 MB RAM, Chaos Calmer is ancient and has many known security flaws. I'd recommend getting a newer well supported device and flashing a recent version that is more secure.

Sorry, I neglected to mention that the Tech' and the TP-Link are on opposite ends of the house, and one's signal reaches where the other doesn't. As for a new router - gladly, as soon as the monetary circumstances improve. This one supports up to 18.06 though, so might as well update. Thanks for the heads-up!

I swear, if this is akin to telling the new user to rm -rf /*...
Jokes aside, I'll report back once I try this.

Never fear, these commands will report information that will be useful for people trying to understand how your system works: ip addresses, routes, network names and numbers, firewall rules etc.

The request means for you to run those and copy and paste the results here.

Ok, this makes some sense. Is the TP-Link plugged in via wired connection to the Technicolor? Perhaps what you want is for the TP-Link to be a "dumb AP" rather than a separate router: https://openwrt.org/docs/guide-user/network/wifi/dumbap

This would put all your devices under the control of the technicolor's network. I don't recommend this myself, for security reasons (you must assume your ISP has full access to both sides of the technicolor device). Nevertheless, it is a way you could set things up.

If possible, I'd rather avoid that - you state the reasons quite well yourself. As to the situation, I upgraded to the newest stable build available, 18.06.1. Lo and behold, the PCs now see each other. The internet connection, however, has flown out the window. Can't even ping the Technicolor from the TP-Link. And very wisely, I had decided not to back up the config. Hooray to my idiocy.

BTW, the ip command wasn't present in 15.05.01. It does work now, though. Still relevant?

Ok, so if I understand your topology it looks like this:

INTERNET 
|
|----- Technicolor -------- OpenWrt TL-WR841N ------ Net A, Some devices wired and wifi
            |
            |
    Net B Some Devices via Wifi, addresses unknown

Under stock config, which you can get back to by flashing the new 18.06.1 and telling it to NOT keep your settings... all the devices whether they're on Wifi or Wired, if they're connected to the OpenWrt device they should talk to each other. Since the Technicolor is using 192.168.1.0/24 network, you should set up the LAN on your OpenWrt to use something else, like 192.168.2.0/24.

If this doesn't work, even after going back to stock and doing nothing but just setting up your SSID and password and changing the net numbering on LAN... then let us know.

Next, be aware that unless the networks are bridged, the Technicolor and the OpenWrt should have different SSIDs because they can not roam between the two networks.

Making Net B and Net A talk together is not a good idea, Net B is "the internet" as far as your OpenWrt is concerned. opening up devices connected to the Technicolor to talk to Net A means opening up Net A to the internet. Going the other way should already work, Net A accessing Net B devices. If not, it's probably because you need to renumber the network on OpenWrt to use something like 192.168.2.0/24

That topology is indeed correct.

Changing the router's IP range worked. Interesting, as I had tried that before upgrading to 18.06.1 with no success. Thank you kindly.

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