Hi
Cant help you with coverage, but we have a T-Mobile dongle, and can either top up per month or day. Its £2 per day, but when you first sign up they give you 30 days for £2.
We have used ours all over the country and not had a problem, but next weekend we are taking it camping, so will see if sat in the middle of a field is an issue.
Jenny, Harley & Mayhem