If I understand correctly, TWRP (and Magisk) are gone after the OTA update, and need to be re-installed:
The key thing here is that it [the OTA update] installs a fresh, fully stock version. Your data gets pulled over, but none of your mods go with it. To get them back, you have to install them all again. There are various ways of doing this, and this isn’t the place to repeat the guides that are already posted on how. Just understand that you have to do it to keep root, twrp, and your other mods.