29 volts too much for a 24 volt thermostat?
This seems like poor design of the thermostat: most household HVAC "24 VAC" transformers are designed to provide that voltage under load, i.e., while drawing the design current, perhaps one ampere. Open circuit (O.C.) voltage of 29 VAC is normal.

That said, it would be easy to reduce the voltage to 24 VAC. Two 6 VDC zener diodes such as the 1N5340BG (~US$0.50 each) soldered in series, with opposite polarity, would bring the supply down to 23 VAC, giving a 1 volt margin. However, I question the manufacturer's claim that the normal O.C. voltage is causing the issue. and doubt that lowering it by 6 volts would help. More likely, the connections are incorrect or the thermostat is defective.