How can I fund my Saxo trading account (base currency USD) from a Singapore bank account with SGD balance? Can I use money transfer service such as Transferwise or Instarem? What's the cheapest way?

As a former Saxo user, sorry, there is no cheap way. Transferwise transfer gets rejected. 0.75% conversion fee is too much.

I switched to TD Ameritrade and Stan Chart

I contacted Saxo and this is their reply from one of the agents:

We do not advise the use of remittance company as Saxo do not allow third party transfer. Funds originating from third party firms may not show your name and thus may not be booked. You will need to provide necessary supporting documents (subject to payments team review, if funds cannot be verified), in the event of rejection of funds, Saxo is not responsible for any fees involved.

So I guess its possible but it will be troublesome.​​​