Have you signed up already? During the signup process, you can request to go with the ONT.
As for 10G capable router, there are quite some choices below S$500.
1) MR users dumping TP-Link Archer BE805 at around S$400 in Carousell
2) SH users dumping TP-Link EB810v at around S$400 in Carousell
3) Asus RT-BE88U (now only available from Asus China), no 6GHz band, many LAN ports
4) Xiaomi BE10000, no 6GHz band, you may need to add a SFP+ module by yourself
There are also other choices like Asus RT-AX89X (you need to add a SFP+ module) which you may find lobangs to bring the cost down.
You can of course go for more expensive router as well if you want the better ones like Ubiquiti UDM SE and Asus ROG GT-BE98.