Mobile Broadband on Ubuntu 9.10 Not Working

Going away for a couple of weeks I had planned to use the mobile phone tethered to the laptop for occasional Internet access. I had used it once or twice before, and although only EDGE (not 3G), it is good enough to check email, browse some news and check some tourist. As an aside, 3G mobile Internet is a contentious issue in Thailand, with very little UMTS coverage, including in Bangkok, provided by the popular operators AIS and DTAC. There has been a prolonged decision about awarding spectrum licenses I guess mainly because of the business and political conflicts between the two government telcos, CAT and TOT, and the commercial operators, AIS, DTAC, and True. Anyway, I was foolish enough to assume the mobile broadband access would work when I got to Chumphae, without testing before I left home. Of course, it didn't, and I'm guessing that it is something due to my upgrade of Ubuntu from 9.04 (when it worked) to 9.10 (now it doesn't). When I plug my phone in to the laptop (via USB), it is recognised and the nice wizard for creating a new Mobile Broadband connection allows me to select my network operator (AIS). However when I connect, the Network Manager almost immediately reports GSM Disconnected - You are now offline. I do have Internet access on the mobile where I can browse some sites, so I checked some Ubuntu forums, but haven't found a solution yet. I will diagnose the problem when I get home, and post more, including my thoughts on Ubuntu "upgrades". Update: I worked out how to fix this - see my solution.