How are you centering the servos? Or what is the procedure you've attempted thus far?

Your goal is to match the servos to the drive pulse your TX transmits with trims and mixes zeroed out. Most TX's do NOT transmit the industry defacto standard of 1500us pulse width (us=micro-seconds). So setting the servos to 1500us is pretty much worthless in this scenario...

If you have specific questions or needs share them here and I will help you work through them.