+1 to be able to put an LTE interface inside a bridge.
But this will not remove the biggest problem, the NAT inside the LTE 4G modem. To remove that limitation we need bridge mode inside the modem, or IP passthrough.
The USB LTE 4G modem i tried, a E3372h from Huawei, can't deliver the WAN ip address through IP passthrough. Router mode is mandatory and DMZ or port forwarding is not available in the modem configuration (through web interface). So in the end we get a 192.168.8.0/24 nated subnetwork from this modem. Very bad when we use a Mikrotik for advanced network management. We end with a double NAT setup without possibility to configure any port forwarding.
The best we could do here, is to flash a modified web GUI in the modem, to give access to port forwarding and DMZ settings. But router mode and NAT will stay anyway enabled. More this flashing is not supported by the modem vendor. This is a terrible solution.
Flashing the E3372h in Stick mode (it become an E3372s modem) , an USB port appear inside Router OS, but PPP connection on this port does not work for me (Mikrotik HAP RB951, Router OS 6.38.5).
The same modem works under Windows 7 and deliver the WAN address through RNDIS interface. So it seems to be a driver problem inside Router OS.
Anyway the ppp connection is really slower than LTE and is CPU hungry. So this is not a good solution technically.
So would it be possible to switch those modems in bridge mode (using QMI commands ?), so that we can get the wan ip (through DHCP ?) on the LTE interface ? Then accessing the modem web management interface should not be a problem, adding a static IP address in the 192.168.8.0/24 range on the LTE interface. I did that for PPPoE DSL modems in bridge mode linked through Ethernet to a Mikrotik port. Adding a static IP to the Ethernet port of the Mikrotik gave access to web GUI even with the modem in bridge mode. The wan address going to the PPPoE interface on this same Mikrotik Ethernet Port.
As it is not easy to modify the routing table in the modem, adding a masquerading rule on the Mikrotik Ethernet port where the modem is connected give access to the modem from other subnetworks without touching the modem routing table. It is even possible to distribute this modem administration IP subnetwork through a routing protocol to get access to the modem Web interface from everywhere in the network.
II suppose it is the same in LTE 4G routers, the modem ip (192.168.x.x) certainly stay available even if the modem is in bridge mode. The only difference is that the modem internal DHCP server is disabled, so it cannot deliver an address for the web interface (if enabled there would be two DHCP servers, a local one and the provider one and they will conflict).
Some 4G LTE modems do allow bridge mode to be configured from their web management interface, for example the Netgear LB1111. But it is a Ethernet port modem, not an USB one.
So it seems definitely possible technically, some advanced users seems to get it working on OpenWRT using QMI commands.
Some modems in Ip passthrough mode seems to use a trick to enable two DHCP servers (local and wan) in the setup. In this case :
A reduced (two minutes) dhcp lease time is assigned to the modem local DHCP server. When there is no wan connection, a local IP address is delivered to the client host to allow for modem administration.
When wan connection is active, the modem local DHCP server is disabled (i suppose) and the client host can get a wan address and gateway, this time delivered by the wan side DHCP server.
See here : https://www.att.com/gen/general?pid=23697
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