Step 1. Apply.
Have you been to your local office? Have you applied?
You will need to establish that you have custody of this child. What evidence do you have? Who is listed at the school as emergency contact or who enrolled the child? Who takes the child to the doctor? Is the child listed on a public assistance grant with the mother?
SSI checks change based on a child's circumstances, living arrangements, parental income. Some parents use the child's SSI as the family money but it is not.
If you have custody, and you can prove it, then SSA would probably consider you to be the right payee. The mother will find out and she may disagree, but it is SSA's decision. And if what you say is true, you have had custody for 11 years, but she is the payee of the SSI and she gives you some or all of the money, I am pretty sure that she says something else when SSA asks.
You were ok with this arrangement for 11 years. What changed? She is not giving you as much money as before?