Now, I want to cash out some profits but the customer service won’t let me do it.
They asked me to deposit money (20% of my profit gain) in to the platform (they said this is for taxes on my capital gain), so that I could cash out my current assets in the platform.
Reading this, this is all I need to know.
That's a scam.
The exchange is unknown and made you look like you've won your trades and got you in profit. Now, that's the typical scam that happens, they will ask you to deposit first before they promise you to allow the withdrawal.
Just accept it that there's no real money that you've made on that platform and they're trying to scam you.