in order for it to work you need your idevice jailbroken, overwise it wont work
if you also have a pc there is another way with downloading data every week
set up process:
setting up miitomo:
log into apple store with your account
if not - download kaerutomo (kaerutomo.ipa) from the site below:
after downloading and installing the app (should be 2.4.0 version) add kaeru apt repository so that you can download the tweak. visit in safari and tap 'add to cydia' (for cydia and sileo) or 'add to zebra' (for zebra). then within your package manager (refresh sources if necessary) search for the 'kaerutomo' tweak and install it.
use this guide
do what it tells you
it should start working
if not - check out these lists
important! this guide can contain errors as im not an apple user myself & it was learned from someone else. for corrections message me in kaeru server