I have an old WRT54 that runs on 10.3, so it's really old. But the box is working, and I want to set it up for a friend as an access point. It is connected by cable via the WAN port to the primary router. The WAN is set up as DHCP client. The lan ports and the wireless interface are in the LAN and configured as DHCP server.
This works fine as long as the cable at the WAN is not connected. I can connect to the LAN by cable and wireless and get the configured IP via DHCP. As soon as I connect the cable from WAN to my primary router and restart the box, now I get an IP address provided by the primary router! That would not be so bad, as the internet works that way. But now I cannot access the Wrt54 anymore, as there is no visible IP address for it. It looks like the box is just transparent.
It is highly unlikely that anyone will remember how to make meaningful changes to the configuration of a version that is almost a decade old!
Even the most updated version of OpenWrt that can be used on that device would have serious security vulnerabilities and would be unsupported in every way.
Recycle the device - it is too old, obsolete, and slow to be supported.
Well, if I may say, if it's all in the same network, strictly as an access point, then security isn't much of a concern, I think.
As for the problem, I imagine it could be that WAN is bridged with LAN. Anyway, you could update it to the recent version available.
Or you could also disable the DHCP server on the WRT54, give its LAN interface a static IP on your friend's main router's IP, and that will hopefully work.