mobile can't be directly bridged to eth, because it do not have layer 2 header.
So LAN_MOB1S1A1 is not missing.
RUTX_R_00.02.03.1 firmware is using arp proxy to accept packets from LAN host.
So when you enable bridge mode, you should get dhcp configuration on dnsmasq with lease of mobile IP, br-lan with proxy arp, and two routing tables 42 and 43.
Modem firmware should not be problem, because it's another independent system.
As much as I tested it worked fine, even without MAC address.
Try to look over logread over ssh, it should show dnsmasq configuration, and proxy arp settings.