The continuing info has been so helpful. You all are awesome!
and for those who might search as I did for current internet news during the Irmaria transition, Here's what I did.
My single-user solution was needed only for iphone and laptop for a couple months:
DONE #1. Switched my AT&T plan from "mobile share advantage 1G" to domestic "unlimited plus", effective immediately with a phone call. The difference in cost was $45/month for the several weeks I will use it, without activation/termination fees. I couldn't just add to my existing plan because existing limit was 1G, and would have cost $15/month more for each extra Gig actually used. (BEWARE!!) Domestic range, plus USVI, Canada, Mexico
Two other options, at slower download speeds of 5-20Mbps:
#2. Add to existing AT&T plan:
"hotspot" for $20/month with one time $25 activation fee. Includes a physical router device (shipped or picked up at AT&T store) to carry around and remember to recharge, which is assigned an extra phone number. But I would have triggered significant overage charges.
#3. Add an iphone feature without any additional device, with existing AT&T plan:
"ATT passport" for $60/month, no activation or termination fee. included unlimited worldwide texting at no cost.
It took asking AT&T lots of questions, especially how existing plan data limits worked differently with each option. Option #1 came out ONLY in the last 5 minutes of long phonecall.
Thanks again for all the practical help.