I tried last year (for Singtel website) ... got only 0.7% rebate. Additional 0.3% if you meet the minimum monthly spend of $300
I think if you use Singtel App to do payment, it may be treated as "online". If it is, it's 2.7+0.3% but I have not try this option though...
I can confirm that my recent payments(last two months) via both Singtel App and M1 App have earned 2.7%+0.3%.