For some reason, this information is not on the open internet, but has to be tortured out of Shirley, the chatbot. Some of it is counter-intuitive (for example, the reset duration is 30 second [sic], not 10 or 15 seconds, which is more normal).
For easier access, I am reposting what Shirley spewed out here:
"Step 1 : Power off and on your devices - Router, Optical Network Router/Optical Network Terminal, Set Top Box and WiFi Mesh - and wait for them to boot up.
Step 2 : The power LED of each device should come on and be solid (no blinking). Ensure the WiFi Mesh’s LED indicator for the 5Ghz and 2.4Ghz is lighted green. If the 5Ghz LED turns red, you should relocate your device as it is out of the WiFi range.
Check internet connection after above steps. If you are still facing problems with your WiFi Mesh, please continue with the following steps:
Step 3 : Perform a (factory) reset on your WiFi Mesh. Located at the back of your WiFi Mesh is a “Reset” hole, take a pin and press into the reset hole for 30 second before releasing.
Step 4 : Wait for the devices’ power LED to be solid, and continue on with the pairing process in step 5
Step 5 : Press and hold the WPS button on the first WiFi Mesh, followed by the second WiFi Mesh. Ensure that you are holding the button for 2-3 seconds, and that both buttons were pressed within 60 seconds of each other.
Step 6 : Once this process is completed, wait for the 5Ghz and 2.4Ghz light to turn green. Then you can check your internet connection again. Relocate your device if the 5Ghz LED turns red as it is out of WiFi range."
What makes things even more fun, though, is that I reset it again and still faced issues.
I called the hotline, and the lady checked a bit and told me that... the IP address at the back of the WiFi router, 192.168.1.254, actually is for the optical network router (ONR), not the WiFi router (despite what Singtel Mesh: Change Wireless Settings & Device Info tells us).
So to access the router, what you need to do is login to the ONR, key in the default password on the back of the ONR, then view the devices on the network, find the IP of the WiFi router and access it to login to the WiFi router using the default password on the WiFi router.