Balance methods
There seems to be a bit of a misunderstanding here about how the frame is balanced. As a human uses the frame, shifts in weight are what keeps it upright. Basically, this is a version of the inverted pendulum problem, which should be convered in detail in any text on nonlinear controls. In addition to the leg servos, you’ll need to mount some sort of weight on an actuated arm above the frame and use that to shift the center of gravity of the robot. Search for information on self balancing (segway-type) robots for basics on accomplishing this. Also keep in mind that bipedal walking is a process of falling foward and then moving the legs under the center of gravity to arrest the fall. Working on balancing first is a step along the way.
Just don’t underestimate how complex your body’s self-balancing ability is - you basically need to replicate the full system, including sensors (eyes, inner ears) and actuators (muscles) along with the ‘brain’ to tie it all together.
Good luck!