Drop-In Mode Fails to Pass Traffic from LAN to WAN Interfaces

I’m looking for some assistance in troubleshooting what appears to be a simple setup.

I have UBR-LTE, for which I would like to bridge the LAN and WAN interfaces and pass traffic to hosts on either interface transparently. To do so I have enabled “Drop-In” mode on the UBR and selected the single WAN interface. I have configured a default gateway, but I’m not actually using that functionality. Here’s the setup:


Both PC1, the UBR and PC2 are on the same subnet.

While I can ping from the UBR-LTE to PC1 and PC2, the UBR-LTE will not pass traffic from the LAN to the WAN interface.

Any suggestions?