For your Asus, usually you may want to carry out factory reset and then set it up again. You may have to use AP mode in order to avoid Double NAT and to make Asus AImesh Ethernet Backhaul to work.
If you were using ONT, usually your Asus router will have Singtel VLAN profile enabled, that may well be the issue for you. You have to disable Singtel VLAN profile.
On the other hand, it makes sense to use the two HB611-SGST if you want to get faster wired and WiFi speed, as HB611-SGST has 2.5G WAN port and 2.5G LAN port, as well as triband WiFi 7 BE11000.