I needed a reliable LTE backup WAN on OPNsense using a Sierra Wireless MC7455. PPP could establish a session, but traffic suffered from FCS errors. Switching to FreeBSD’s native umb(4) driver and MBIM solved the transport problem.
The remaining issue was OPNsense’s handling of the dynamic point-to-point gateway assigned to umbX. I wrote a watcher that automatically maps each umbX device to its OPNsense interface and gateway, then keeps the gateway synchronized with the current MBIM peer.
The solution supports multiple umb interfaces, does not depend on optX or gateway names, survives reboot, and has been tested with failover and failback.
Full source code and installation instructions are available on GitHub.