L293D and PWM speed control
I think you have slightly misunderstood the wiring schematic. Typically pin 8 and 16 are the main supplies that will be driving the motors.
The PWM signal from the arduno will often be connected to pin 1 or pin 9, depending on witch H-bridge you are using
TI's driver comes with a wiring schemattic you can use:
https://www.ti.com/product/L293D