have a one digital box with one tv and one converter with other tv main tv works but the one with converter can't change channels

You should really get FiOS!

I'm a Xfinity Rep. bring in the faulty equipment and we will exchange it free of charge

Drop off your equipment at the nearest comcast, Xfinity location and swap for new gen digital tuners. No charge.

Try a reset from the main box. Hard reset the additional room. Chat wth Comcast service.