Please confirm … if you press Vol + all the way through rebooting (from the moment the screen goes blank to the moment TWRP appears) … this doesn’t work?
If in doubt, you can do this anytime.
Well, there’s always booting TWRP without flashing it …
https://forum.fairphone.com/t/howto-use-twrp-without-flashing-it-e-g-for-backup/23747