Make sure you don't have any iptables rules blocking the traffic. Might be easiest to just let anything through these interfaces while you are testing, but make sure to fix that before you put this into production.
An "LTE bridge" is nothing more than an ordinary router with proxy arp enabled on the LAN. And most often, for convenience, a DHCP server config generated from the values assigned by the LTE network. There is nothing magic to this.
You might want to tune the ARP and DHCP timeouts based on the fact that there will be only one IPv4 address to play with on the LAN side, but that limitation is pretty much the only thing special about the setup.
It's obviously even simpler for IPv6, where you will have 2^64 addresses and therefore can serve as many clients as you like. You can even use a couple of those addresses for the router.
Just for information, someone on a German forum, arrived to make the bridge mode to work in MBIM mode, by slightly modifying the source code of dnsmasq.
Apparently it works.
Hey Bjørn, thanks for having a look. Here's what I have at the moment, still cannot access internet from the host, which I have configured statically with the IP configuration of wwan0, and which is connected to eth1.
I have stopped/disabled mwan3 and firewall for testing.
root@OpenWrt:~# ip rule
0: from all lookup local
5042: from all iif eth1 lookup 42
5043: from all iif wwan0 lookup 43
32766: from all lookup main
32767: from all lookup default
root@OpenWrt:~# ip route
default via 120.157.99.120 dev wwan0 proto static src 120.157.99.119 metric 50
120.157.99.112/28 dev eth1 scope link
120.157.99.112/28 dev wwan0 proto static scope link metric 50
192.168.1.0/24 dev br-lan proto kernel scope link src 192.168.1.1