I cannot get a WAN IP with OpenWRT. It works fine with LinkSys stock firmware. I can also get WAN IP with a different router like USG. Issue is only with OpenWRT firmeare. I have done the basic trouble shooting ( turn off the modem, router, different versions of firmware etc ).
Issue is seen with David's build also. ( I see the same issue in DD-WRT also ). I have used the OpenWRT with the same router a while ( 1 to 2 year ) ago ( Don't remember the version ). It was working fine at that time.
Mac address of the router remains the same between Stock and OpenWRT firmware. Stock firmware always works fine.
Note: I have disabled wifi in the LinkSys ( Issue is seen with Wifi enabled also). Using Unifi AP as access points. Currently using USG instead of LinkSys.
I just went through that. I don't see any issues with cable modem. It works with stock firmware of the same LinkSys router and also with a different router like USG. Issue happens only when I install OpenWrt ( or DD-WRT. I tried both ). The default settings ( and suggested settings) fails to get the Wan IP.
Connecting WRT1900ACS to LAN port of another router is just to demonstrate the WAN port is working with OpenWrt, but there is clearly some issue when used a cable modem & ISP.
I agree that a good test is to ensure that the WAN port (physically and logically) is working properly by connecting another router's LAN > WRT1900ACS WAN. Make sure they have different LAN address spaces (when traversing across NAT or really any routing layer, it is mandatory to have different subnets on each side).
Also, make sure you're running a completely default configuration (aside from possibly the LAN address space) -- you can reset to defaults within LuCI or use the firstboot command when you ssh into the device.
If the WAN works properly in the above configuration, it should be possible to get it to work on the cable modem. Keeping in mind that you may need to power-cycle the modem several times (and sometimes even call the cable ISP to ask them to release any associated MAC address bindings), I also just learned about the fact that sometimes it will work if you just wait a while because of the DHCP request timing. Check out this post over on the UI forums, and importantly it links to this other thread that actually has a dump of the DHCP requests with timestamps.
I have had similar issues getting a WAN IP from my cable modem -- I've always gotten it to work in the end, but next time, I'll just set a timer and wait. If that works, it will save me a lot of time and frustration knowing that I just need to be patient.