Nighthawk AX5400 will not connect to internet

Model: Nighthawk AX5400 (RAX54S)

I’m encountering an unusual issue with (2) brand new AX5400 (RAX54S) routers being unable to pull an IP from the ISP’s DHCP server. Please note that I’ve done the traditional/basic troubleshooting already (power cycle, factory resets, etc). This is something more nuanced than the ol’ off and on again can solve.

The ISP is a community fiber provider in a rural area, the only in-house equipment is a Power over Ethernet injector that provides the internet cable for the router. I have confirmed with the ISP that the customer can use whatever router they prefer. I’m currently trying to replace a Motorola MR2600 router. The internet works fine with this router. I can also directly plug my laptop in to the injector and it pulls an IP address fine.

So why won’t the Nighthawk? I have experienced the same symptoms with both (brand new) routers. It does work if I leave the old router plugged in and connect a cable from the Nighthawk to one of the LAN slots, so it can pull from DHCP on these routers (just not from the ISP apparently).

I have tried all (3) available firmware, no difference. (RAX54S-V1.0.5.106_2.0.70, RAX54S-V1.0.10.110_2.0.75, RAX54S-V1.0.11.112)

I’ve also tried cloning the MAC of the old router, setting a static IP, no help. Disabling the WAN security features on the router did not do anything (Disable Port Scan and DoS protection).

Anyone with ideas of what to try next? It seems to be a firmware issue, possibly security-related? Thanks for reading.