This exchange is renowned for this, they selctively scam their customers, meaning you could be using the exchange all right, but then you might just deposit a large amount and they steal your funds. That said, i am surprised that you sent that large amount all at once, if you want to exchange a large amount of money in any custodial platform, you should divide your deposits into smaller amounts and deposit them that way.
The only thing to do now is to continue calling them out aggressively anywhere you can.