Take it with a grain of salt (http://www.trekbbs.com/threads/rumor-the-show-takes-place-between-tos-movies-and-tng.280166/), but Devin Faraci claims:
After the TOS films, before TNG (http://birthmoviesdeath.com/2016/04/13/the-new-star-trek-tv-show-will-be-set-before-the-next-generation)