I recently upgraded to a DrayTek Vigor 2860n+ router which can run two USB 4G mobile broadband modems simultaneously. I have two unlocked Sierra Wireless AirCard 320U modems with Virgin Mobile SIMs. The modems are positioned on a high window sill where the signal strength is optimal. To achieve this, I’ve used USB extension cables. External antennae can be attached to the 320U, which would have been a good, albeit expensive, alternative. On the window sill, the signal strength is typically over 60%, but 1m below the window, it’s only ~30%.
For the first few days with this setup, the router was terribly unstable. It would barely go ten minutes before the internet connection dropped out. The router itself would sometimes crash and reboot. After testing different settings and configurations and even a call to DrayTek support, the problem was found: one of the USB extension cables was too long. One cable was 1.8m long, and that worked fine. The other was 3.0m, and the voltage must have been dropping, causing the modem to misbehave.
So, I purchased another 1.8m cable, identical to the one I already had, but the problem persisted. So, I ordered yet another one: a 1.2m USB 3.0 extension cable. USB 3.0 supports up to 900mA of current compared to 500mA for USB 2.0, so I was hoping the new cable would work better.
And it does. The new cable is thicker, too, which probably helps. Yesterday, the router ran flawlessly for over nine hours, and neither modem dropped its connection for the entire time.