My iPhone can’t see my Beats Pill on the Bluetooth menu

That sounds like an issue with the speaker itself.

If holding down the Power button for 3 seconds does not place it in Discovery mode (the light next to the button should pulse when in discovery mode) or if it still won't appear on either iPhone's bluetooth settings even when the light is pulsing, the BT chip may be malfunctioning.

You might have to get it looked at at an Apple Store or authorized service provider.

click here ➜ Beats Repair and Service - Apple Support

Is the Beats Pill charged and set to paring mode?

Has it been paired to something else previously?

click here ➜ Set up and use your Beats Pill speaker - Apple Support