It's wired. I have no idea.

I have same problem. Router cannot recognize modem (ZTE MF637) in GPRS coverage. Same router and modem works well in 3G coverage.
Will somebody help us?
Edo

I have just sent info to tech support with similar problem maybe.
I have 3220 too, with the latest firmware (V2). Trying to run the network with ZTE MF637 USB Modem. It keeps dropping the modem also, or when it connects finally to the internet (status page) after a few bytes of transfer (ping to web IP doesn't even responds yet) than it freezes. I have to restart the router manually. Than the same again, again, and again. Tried a lot of things, with no positive result.
I think in our location we have poor 3G signal, and the modem switches automatically to 2G. So maybe my problem is the same as hogl and kirdas wrote...
I'll try the modem/router in 3G coverage and post back the result...