Two years ago, the annual Under the Radar festival (which showcases international, experimental and multidisciplinary theater) was unexpectedly canceled by the Public Theater, its longtime presenter, due to financial issues. In response, the festival was quickly reconceived as a citywide effort involving several other theater companies, allowing it to move forward. The festival, now in its 21st edition, returns this month with productions at theaters across the city from Jan. 7 to 25.