Should i start investing in Nikko AM Singapore STI ETF using POSB RSS ?

If you want to invest in a poor-performing index like STI when you can invest in other index like S&P 500, you might want to reconsider your options

Investing in a security/buying a financial product in order to get a higher interest rate for your savings account is a bad idea. Banks always have the final say when to unilaterally change their interest rate policy without incurring penalties

I guess no one can really time the market but if you check out the prices since march, STI ETF has gone up by a significant margin. However since you're considering POSB RSS which I believe is the invest saver plan, you should have less worries over this.

DCA will aid you in averaging out the price whether it continues to go up or down. Most importantly is to continue investing over the long term.

Lastly, with much uncertainty currently, I would say STI ETF is one very safe and stable option. Again, try not to time the market :)