Depends on your speed/bandwidth needs.
For low usage (email, light browsing) tethering to cellphone (Android works, don't know about iPhone) is an option. Clients can connect to cellphone directly (wirelessly) or you can connect your router to the cell phone wirelessly and use existing wiring. As far as providers go I have an experience with PagePlus (»www.pagepluscellular.com/
) and T-Mobile prepaid. To keep PagePlus account active will cost you $10 every 4 months minimum ($30/year). Or you can just activate it as needed via their website. Their $55/months plan includes 2GB of traffic and they only support 3G phones. For T-Mobile tak a look at this: »prepaid-phones.t-mobile.com/pay-···ne-plans
(note that only first 200MB/day is 4G on $3/day plan)
Another alternative is this: »www.freedompop.com/
Of course keep in mind that in case of disaster any service can become overloaded and/or unavailable.