What is the difference investing in S-REITs as a diversified portfolio offered by a Robo-advisor (specifically OCBC's RoboInvest: Singapore Stable REITs) versus a multiple REITs on the stock market?

Roboinvest will reinvest the dividends from the portfolio of REITS or stocks. Roboinvest also picks the REITS/stocks for you, leaving the research and analysis to the bank's robo setting.

Occassionally, the robo will rebalance the portfolio for you.

If you pick and purchase multiple REITS yourself, the dividends are credited to your account. You would have to reinvest them yourself. Of course, you can create a similar portfolio as the ocbc robo but you would then need to monitor, research and track the individual stocks/REITS.