There was this question here the solution to which was to boot into TWRP before booting android, so that TWRP isn't replaced by Android Recovery. I can press UP+Home+Power and go into recovery - I've not had an issue with that. But the trouble is that rather than showing TWRP it shows an android logo and it says it "updates the system" and then what comes up is Android Recovery instead of TWRP.
I'm using samsung's SM-g532g (galaxy j2 prime). It does not have an official TWRP apparently, so I'm trying the TWRP that this site suggested. I've tried it many ways: I've booted into recovery right after flashing TWRP, removed and replugged the battery and then booted into recovery, etc.