So you got a cutscene with both barrels of explosives? Only one should have been needed, with the other intended to optionally clear a short side branch of the path a little after the main cave-in.

You could try going to Broken Valley and/or the fjords, then return to Sentinel Island and see if getting the game to reload the area would help. Normally I'd recommend first loading an earlier save, but since you don't have one (did the autosave get overwritten?) your latest one will have to do.

I'm not sure what else might get the game to finish the cutscene or let you place more explosives, short of emailing support.