5G Modem w/eth port IP Passthrough *TO* OpenWrt issues

Hi All,

I've been pulling my hair out a little with this. Can't work it out.
I'm using a ZTE MU500 - Manual Here - I've disabled DHCP, disabled NAT, and enabled IP Passthrough on this device.

I'm using a Netgear DNGD3700 v1 with the latest OpenWRT image (21.02.2) - basically with the default settings OpenWRT as flashed via .chk using the original Netgear admin panel. I've been doing all my admin since then via LuCI.
EDIT: Clarity: I've changed the router to be on, with the modem on

The single ethernet port on the MU500 (which works for providing internet to standard devices when the modem is being used regularly out of the box) is connected to the WAN port on the DGND3700.

No matter what protocol I use for WAN (eth0.1, LAN bridged to eth0.1), or what settings, I can't seem to get internet working on devices connected to the router, either wired or wireless. The router itself is also unable to run diagnostics (no ping, no traceroute)

None of the existing topics here that I can find, or info on the wiki seems useful for this particular use case (5g modem with eth, passing through to openwrt), but I feel like this should be fairly straightforward. I'm at the point where it feels like the MU500 is lying to me about what it is actually doing when I check/uncheck the relevant boxes.

If someone could just tell me what WAN interface settings I should be using here I would be forever grateful!

you dont have to disable dhcp on the ZTE. even if you use ippassthrough, you can't disable the dhcp on the bridge interface.
So you must set dhcp client on the wan on openwrt.
If ZTE router ip have 2 interface, check which one they are, example if there is no 2nd interface, wwhit ippassthroug you shouldn't be able to reach it after you set ippassthrough, so check better.
Once you have the ZTE ip range, use a different one for openwrt lan.
example ZTE=, you set for your openwrt lan.
Also better set DNS on wan interface of your openwrt.
let me know if it works or you need more info.

Well done, unfortunately, basically you cannot disable nat on the device which have direct acces to the internet, or you wont communicate anymore. What ippassthrough does, is sort of relay it. Now not every devices works the same, but by it's nature, using ippassthrough, bypass nat, but the device must have nat enabled anyway, or it have nothing to "passthrough".
Happy to be wrong, each device works different, but any ippassthrough works the same, you have no gain disabling nat on the device.

Unfortunately I still had the same issues when I still had NAT enabled on the ZTE. The router either didn't want to accept the passthrough, or the ZTE wasn't actually passing through.

Hmm. Okay. I guess I'll try this again and get back to you.

Just to clarify, ipbridge use dhcp when it's dynamic. to use a static bridge, it should support static ipbridge, but at that point, you would need a static public ip assigned to you, and a static fixed gateway. But, as you know, mobile broadband rarely can have a public ip, let alone a static one. Thats why you must set DHCP on on your ZTE device, and set openwrt wan as dhcp client. This is the reason you cannot disable DHCP when using ipbridge. Also, probably you can disable nat, on the ZTE, because anyway openwrt Wan gets the public ip, thats where the nat bypass comes in. But you must enable nat on the openwrt Wan, which means enabling Masquerading on the Wan zone firewall.That must be on in this case.

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