My colleagues and I have developed a fairly sophisticated pronunciation app with voice recognition and feedback grounded in the Color Vowel approach to teaching pronunciation.
Blue Canoe is the app, and it is currently available for FREE to learners and teachers. (The app is designed for adults but is interface-appropriate for high school and even middle school students). You can learn more about Blue Canoe here and request free licenses for your students here.
Color Vowel is a pronunciation teaching method developed over the last 20 years by me and my co-author, Shirley Thompson and shaped by the input of thousands of teachers and learners around the world. You can learn more about Color Vowel here or sign up for a free introductory webinar here.
As for online pronunciation teaching, both Color Vowel and Blue Canoe support remote learning. The app can serve as a powerful 'mobile workbook', and the method lends itself to teaching via video conference.
Happy to answer questions!