You need to call Singtel to re-provision IPV6 if you are using ONR. The configuration may have been lost somehow.
I am on Singtel Unlimited Fibre (never bothered renewing, so continuing to enjoy benefits) including self-configuration.
If you can change the settings yourself: (take note that some IPV4 sites may malfunction, such as singteltv.com.sg

)
Connection type : Tunnel 6rd
DHCP Option : Disable
IPv6 Prefix : 2400:d803::
IPv6 Prefix Length : 32
IPv4 Border Router : 202.166.127.6 (6rd.singnet.com.sg)
IPv4 Router Mask Length : 0
Tunnel MTU : 0
Tunnel TTL : 255
IPv6 DNS Server 1 : 2400:d800::1
IPv6 DNS Server 2 : 2400:d800::2
IPv6 DNS Server 3 : 2001:4860:4860::8888
Enable Router Advertisement : Enable