Any chance of crossflash to open it up?
No one bothers to try for SingTel customized routers like the following.
Singtel Mesh Router, Arcadyan
Singtel AC Plus, Arcadyan
Singtel AC Elite (AC1900), Arcadyan
AirTies Air 4960R, Airties
Wi-Fi 6 Router RT5703W-D17, Askey
Wi-Fi 6 Router (WG660242-ST), Arcadian
The ones made by Arcadyan will probably be not possible at all as they seem to be fully customized by SingTel. I believe it is the same for Askey RT5703W-D17.
Airties Air 4906R seems to be a model not only uses by SingTel (customized by SingTel) but also by other ISPs. So maybe there is a way in theory.
Still why bother? None of the SingTel issued router is good anyway. Just use them as an AP if you are not with SingTel. Even if you are with SingTel, it is also better to upgrade to a better wireless router (ONT users) or AP (ONR users).