My users have been getting "gateway error" or "server not found" errors and my host told me to do the following:
You need to add A records for ns1 and ns2 to the zone file for singtel22.com
Can anyone kindly tell me how to do that? I'm using CPANEL/WHM. Thanks.

In WHM, under Dns Functions click on "Edit a DNS Zone". Select the domain and then add ns1 14400 IN A xxx.xxx.xxx.xxx and
ns2 14400 IN A xxx.xxx.xxx.xxx where xxx.xxx.xxx.xxx is the IP.