Check the DOM to see if it is within a frame, or some container that is causing problems. If you cannot access the radio button by value you can reference it by something else, like index. Find the radio button's container, and reference it by the Nth index. So if your button is the second button in table 12 of table 1 you can hone it down like this:
BROWSER.table(:index, 1).table(:index, 12).radio(:index, 2)
Or if it's the 2nd radio button on the page (always) then...
BROWSER.radio(:index, 2)
You can also reference a radio button by other things:
BROWSER.radio(:value, '3G')
BROWSER.radio(:name, 'network')
BROWSER.radio(:id, 'isc_1HV')
For further help I'd need to know if you can reference other element in the same container on that page. For info on how you can reference an element, see here: HTML elements supported by Watir
And if you get totally stuck, you'll need xpath. But this should be a last resort, frankly.