Depositing money into trading212 is easy. But withdrawing to BigPay can be costly. BigPay isn't a multi currency cash account. It can only hold MYR currency. Any withdrawals/refunds from trading212 to BigPay may incur TT fees / currency conversion fees (as mentioned by Ramjade). These conversion fees (for the refunded amount) may not necessarily follow BigPay's rates.

The second problem is, withdrawing money from BigPay is troublesome and costly. Recently, BigPay has reduced withdrawal limit via bank transfer to RM 1,000 per withdrawal (maximum RM 2,000 per month), with RM0.50 fee per withdrawal. If you want to withdraw more than RM 2k from BigPay, you have to visit ATM to withdraw in cold hard cash. There is a RM6 fee per withdrawal. There is a limited amount that you can withdraw, depending on the type of ATM bank, location of ATM, day of week, and even time of withdrawal. Very few ATMs in Klang Valley allow withdrawals of RM 4k. Most ATMs have a RM 2.5k or RM3k withdrawal limit. Good luck hunting.

If you still wish to proceed with Trading212, your best bet is with transferwise. You need to get a multicurrency cash account. But this feature isn't offered to Malaysians.

Leave your options open. Do not limit yourself to deposit and withdraw via BigPay only. Stop wishful thinking that Trading212 will offer more withdrawal options in the future. You need to prepare for the worst, and hope for the best.