LOL. Based on a thread over in the other place, it sounds like it may well be in Tokyo proper.
If you want to email them about coverage in Hokkaido and the places you are going, the email that they responded (pretty quickly) to a couple of questions I had is
info@globaladvanced.com. We were in Tokyo and points south as far as Hiroshima/Miyajima and coverage was peachy.
FYI, you first make the reservation, and they then email you and confirm the order details. That email then has a link that you use to process payment. (I would have been confused as to why I wasn't asked for payment upfront if my brother hadn't mentioned this.) I had problems with 2 of my 3 credit cards (the credit card company rejecting foreign transaction), but a third did go through. This is all by way of mentioning that I was in a bit of a panic because I left it to less than a week before I was heading out. My brother was more on top of his planning (as it seems you are), so hopefully you won't have issues. He did his about a month to six weeks in advance. Supposedly you also get a discount for liking them on FB. I did, though wasn't really sure I got the discount; my FB name is a pseudonym and so doesn't match the credit card. It was so much less than the international data plan I usually fork over for that I didn't check or quibble.
FWIW, it also was nice having it delivered to the hotel. After the long overnight flight, the last thing I wanted to do was find the pick-up place at NRT even though I did have a guide meeting me outside of customs.