2830Vn+, latest firmware, with WAN1 as main, and WAN 3 as fallback. USB 3G modem, Sierra 875u.

Situation is that the setup worked perfectly -including fallback testing- for a few weeks, then developed a problem where if the USB modem is connected before the router is powered, the router goes into a cycle of continual reboots at about 30sec intervals. If the USB modem is connected after router startup, then no problems, and all services including 3G work normally until the next reboot.

Since the router is in a locked room, this situation is unacceptable.

I at first thought this was a USB overload problem like used to happen with early USB broadband modems, but a test with a powered hub eliminated that. It seems more like it is some kind of router firmware bug.

Hard to see why the router should be addressing the 3G modem at startup when it's not required anyway, but I guess the Draytek coders might be able to shed some light on that. In any event, even if the modem appears as an unrecognised USB device, that surely ought not to cause the router OS to crash.

I note that while the 875U is listed as a compatible 3G adaptor, it is not specifically approved for the 2830Vn+. Unfortunately, trying to source a 3G modem that IS listed for this router AND which isn't simlocked to some unwanted carrier service is taking hours of searching. At the moment it's uncertain if a different 3G modem would cure the problem, anyway.

I'd thought of using an Ethernet 3G modem on WAN2 as a potentially more reliable arrangement anyway, but they are even harder to source. Any ideas?

-As an update, it seems the problem only arises on bootup after a power outage. Rebooting from the browser interface causes a normal (single) reboot.