Just to contribute data point for Indonesia Roaming,
IndoSat (On their website) but in reality I only can connect to Indosat Ooredoo Network
Both apperantly are owned by different stakeholders although same company
Medan - Generally OK, you get 5G but in reality it's 4G speed and good enough.
Lake Toba - Lot's of dead spots, if you are not within a large town, no signal. Where's Telkomsel (Zym Roaming) has signal everywhere and useable. Some smaller towns I get "E" Edge which is below 3G, it's unusable even for WhatsApp
Remote villages leading to Berastagi, Medan direction, spotty as well, if you are travelling on those mountaineous region, basically the ring road outside Samosir, best to have offline maps saved. Many places I experience non existent network coverage.
Batam - Generally OK, but some indoor malls speed can be very slow.