Netgear WNDR3700 no WAN port link

Unable to get a link established through the WAN port on a Netgear WNDR3700v2 running OpenWrt 19.07-SNAPSHOT r11073-84154574c8. I've tried connecting via a known good ethernet cable to both a cable modem and a LAN port on a different router.

In both cases, the WAN port light on the WNDR3700v2 periodically flashes however there is no light on the connected port on the other device it's connected to. I can further see through the LuCI Network > Interfaces page that the TX count periodically ticks up while the RX count remains at 0.

With the router running Netgear firmware, I can achieve a link through the WAN port and get an IP address on the port via DHCP.

I have been able to provision one of the WNDR3700v2's LAN ports on a VLAN and achieve a link to my cable modem and use this workaround to have an internet connected network through the router. However, this isn't a long-term option for me as I need the router's 4 LAN ports for LAN devices.

Further details in case they're relevant:

This behavior (no link through WAN port) is the same if I flash the router w/ other non-OEM firmware. I've tried another OpenWrt build as well as a dd-wrt build and a Gargoyle build. So far, only the OEM Netgear firmware allows for a link through the router's WAN port.

Seeing another similar sounding post/support ticket about someone's issue connecting a router to a Virgin ISP's modem, I have tried leaving my modem powered off for 20 mins or so, with no cable plugged into its ethernet port. I then powered the cable modem on and power-cycled the Netgear router, waiting a few minutes for both devices to boot/initialize, then connecting the ethernet cable from the Netgear's WAN port to the cable modem. No change in results.

I have tried deleting/disabling the Netgear's WAN6 interface, in case that might somehow make a difference.

I'm currently linking to the modem just fine through an older Linksys (running Tomato) router's WAN port.

I have tried "spoofing" the Linksys's MAC address on the Netgear's WAN interface. No change in results.

Any help or suggested next steps appreciated. Is there a way to verify basic functionality of the Netgear's WAN port to receive packets? E.g. something like ping-ing an internal IP address or looback device? Is there logging I can enable/look at to get an understanding of what might be going on?

Thanks in advance for any and all help. I've tried to make this plea for help general but happy to provide further details if needed.