On your Sikkim trip, get a North Sikkim sim card from Airtel or BSNL (Lachen and Lachung). They are the only service providers in North Sikkim with connection. JIO does not operate in North Sikkim. Vodafone provides connectivity as well, although it isn't as good. Other parts of Sikkim, such as Gangtok, Namchi, and Pelling, have no problems with connection, and Jio also works effectively. For further information, contact your service provider. There will be no network connectivity at Nathula Pass or Changu Lake.