If you have an Indonesian SIM card from Telekomsel, you can use your phone 24 hours a day but no other provider works on the island. If you have a SIM card, your internet is going to work but only edge, so you can receive and send emails or use what’s app but to use Facebook is not funny.
We have solar energy on the island and the old generator is still working as well. Which means maybe not for 24 hours a day but we have electricity. It is not a problem at all to charge your electric devices but in your bungalow the light is still romantic by petroleum lamps or torch.
You should cary cash with you. The German DKB Visa card and Mastercards may work in Singkil but the safest way is to take money from the bigger cities around.