It costs between $25,000 and $50,000 to build a tennis court. The extra cost for a few webcams and a bullhorn should be less than $1000. There are a lot of great tennis players and coaches in India, Eastern Europe, and Latin America. Instead of hiring an American teacher for $40-80 per hour, why wouldn’t someone hire an overseas tennis teacher for $5 per hour? The teacher could watch both players and give instructions through the bullhorn, e.g., “follow through”, “watch the ball”, “step with the other foot”.
Bonus: the overseas teachers could also serve as impartial line judges and scorekeepers. At a lot of tennis clubs, people are paying $20 per hour for court time so the $5 extra cost of the teacher/judge would not be significant.
You would think that this idea would be popular for almost any sport in which players stand in predictable locations, e.g., near golf tees, yet I don’t think that the service exists.