If you have an Indonesian SIM card from Telekomsel, you can use your phone 24 hours a day but no other provider works in this area. Telekomsel offers 4G, but not everywhere and not always.
You should cary cash with you, it is possible but not easy to get money on the islands and villages. There is an ATM in Singkil, but it is sometimes out of work. The safest way is to take money from the bigger cities around.