Get an Airtel or BSNL sim card for North Sikkim (Lachen and Lachung). They are the only service providers that have connectivity in North Sikkim. JIO doesn’t work in North Sikkim. Vodafone also has connectivity but not that great. Other regions of Sikkim like Gangtok, Namchi & Pelling have no connectivity issues and Jio also works well. Check with your service provider for more details. At Nathula Pass & Changu Lake there will be no network connectivity.