You can't do it from Russia. I don't know why you'd go back there.
You'll have to go to Warsaw, Poland to apply. They have a Federal Benefits Unit at the Embassy there, where you can do the SSN.
But you'll need the passport first, so that takes like 10 days to get. At least it took me about that long via the Embassy in Moscow in 2021.
Then they'll make a certified, apostilled copy of the child's CRBA and passport when it comes in, and you'll use that for the SSN.
Although, since they have the Federal Benefits Unit on site, the process may be more streamlined (I'm describing the process of getting the CRBA and passport in Moscow and having to go to Poland just for the SSN, which I didn't do. I applied during a trip to the US with those certified copies).