AC Plus has two major problem.
1) weak routing performance, need to reset from time to time in order to have no disconnect
2) weak wireless coverage
Your Askey sorts out Issue #2 but not Issue #1.
Singtel AC Plus: AC1200, relatively slow CPU
Singtel Mesh Router: AC1900, faster CPU
Asus RT-AX86U: AX5700, much faster CPU
If you do not have Wifi 6 device, then just go with the base plan and try to get the Singtel Mesh Router for free. If they do not give you the Singtel Mesh Router for free, it is still easy to buy the Singtel Mesh Router from Carousell for about S$30.
And in any case, try to negotate with Singtel on the pricing front. S$39.9 per month should be achievable.
Thanks for your suggestion, they are very helpful!