1S mode in the speedo only makes it reduce its cut-off to 1S voltage. The voltage regulator in the speedo needs between 7.2v and 11.4v (2S to 3S) to provide its 6v output to the electrics.
Booster is easy to use, RC Dynamics is the best in the world. It will give you the volts you need. 12th Savox servo is reckoned to be better than the larger ones, but still a risk. Futaba/JR is the best servo for 1S. HTH
