What are the (correct) steps to use a WAN port as standard LAN? I need another port on the cheap, and already have another device upstream to my AP doing the routing so don't need a dedicated WAN port.
I thought a simple bridge between LAN to WAN would have done the trick, so I added WAN/WAN6 (eth0 on my device) to the default bridge (br-lan, so the one between LAN and WLAN), but after doing that pings still fail to a device connected via Ether to the WAN port. I have verified that the device responds to pings when I directly connect via Ether without the AP (the downstream device I'm wanting to plug into the WAN port is another switch so I've done this ping test with my laptop and simple patch cable directly connected).
Next, I thought the default VLANs that separate LAN from WAN were the problem, so I flattened the Switch to a single VLAN (deleted VLAN_ID=2, so only VLAN_ID=1), but that caused all connectivity to my AP to stop. I ended up having to re-flash OpenWRT via TFTP and restore from backup to get connectivity to my AP.
All I'm looking to do is connected a device to the WAN port and have it communicate with the standard LAN ports, so what am I doing wrong?
My AP is: TP-Link AC1750 C7 v2 running OpenWRT v19.07.2
That't certainly up to you, but say you have an issue with your main router. you won't be able to access the AP (and that can in turn prevent you form accessing any other devices on the network.
I tend to use static leases for all my devices for the same reasons you mentioned, except routers and APs (and possibly servers if I have any); these are significant devices that should have their IPs fixed regardless of anything else.