Remote Controlled Self-Balancing Robot - how to mix control input with IMU input?
The PID get's a set point from you. For a standing still robot this a basically an angle of zero in respect to the gravity direction (which variable you use for the PID depends on your actal implementation, but it always be something derived from the angle to gravity direction). For driving forward, the robot must lean forward. So you set the new angle set point to let's say 2 degrees. The PID will try to hold that value by driving forward constantly, with a speed depending on the set angle.

So, via the remote control you simply change the angle setpoint.