The .SG domain names have specific requirements for nameservers set by the registry.
Maximum Length of nameservers
185 characters, not including domain name
Minimum # of nameservers
Maximum # of nameservers
If the DNS is not resolving and there are no apparent problems, do a Whois lookup at the Registry. If there are any pending contact verifications, the Registry will not allow the domain name to resolve (pending contact verifications are daily instructional emails from the registry to the the admin contact).
You may want to check for this to determine if it is an issue for you.
TLH - GoDaddy | Community Moderator Supporting you at x.co/247support