PEG 4000, sold as Macrogol. It dissolves in water, has electrolytes added.
It works in about 12 hours. It is a stool softener, not a laxative. Harmless, non-habit forming.
It functions by drawing water into the colon, making the feces soft.
IIRC 500 baht for 20 sachets. I use it if the bowels have not moved for a couple of days.
Nothing wrong with high fibre, but if laxatives are included in the formula it can be harmful long term.