I'm not sure how Nickel decides which regulatory domain to use, but I'm assuming there's a geoip matching handled at some point of the signin/sync process, and/or based on your account's payment method.
It's stored in the config (`.kobo/Kobo/Kobo eReader.conf`) under `WifiRegulatoryDomain` in the `ApplicationPreferences` section, value is an ISO 2-letter country code (e.g., FR in my case).
And I can definitely access those funky weather radar bands this way
. (With the usual caveat that scanning for them has to honor a delay or some such nonsense, making them an awful choice in practice ;p).