Satchels are weird, they need to connect with fire to ignite I think. It is much easier to buy/steal bombs from duergar and use them instead!
Yes, with the satchels you need to drop them by the rockfall then send a fire cantrip, fire arrow, or other fire damage spell or item at them to ignite them. I guess chucking a torch or candle would work in a pinch!
I quite enjoyed just breaking the wall down with my druid's wildshape Deep Rothe this turn, but that's an option only viable for moon(?) druids and ... do rangers get a Deep Rothe companion? To be honest, I don't think I've ever checked, let alone recently, if the rubble is vulnerable to other force damage such as Wyll's eldritch blast.
