Go to the Child Tax Credit Update Portal on the IRS web site and unenroll ("opt out") from the advance payments.

The $250 payment that you received is not a stimulus payment. It's an advance payment of the Child Tax Credit on your 2021 tax return.

You ex will get the missed payment(s) when he/she files  his 2021 tax return in 2022.  You will have to pay back any incorrect payments, when you file your 2021 return.  

while you are opting out , the other parent should be opting in.  This is optional.

If the other parent makes too much AGI, maybe you should take the dependent.

I am not even sure how to opt out? What is the amount of the AGI?

I am not sure how to notify them that I am opting out and my ex is opting in. What is the AGI amount?

You really should look at the IRS website where it says "Get Details on the Advance Child Tax Credit" .
All your questions will be addressed.