To make it simple, you can not connect multi routers behind M1/Starhub ONT, you need a router behind the ONT. Whether the router needs to be in the DB box is another story.
The full answer to your title question is more complicated since you also mention ONR.
1. ONR -- it is a router by itself.
1) Unbridged ONR for single network (Singtel plans, WC 2.5Gbps plan, VQ 2.5Gbps plan, Starhub 5Gbps/10Gbps plan if you choose to go wtih Nokia all-in-one ONR) -- multiple ports working (usually same network), no need another router inside DB box usually.
2) Singtel 1+1 Gamer Bundle plan, ONR -- one ONR LAN port officially bridged by Singtel, the other three ONR LAN ports not bridged, two seperate networks, no need another router inside DB box usually.
3)
Unofficially bridged ONR (not supported by the ISPs) -- either one port working or multiple ports working depending on the configuration.
Starhub Nokia ONR bridged for 5Gbps/10Gbps -- usually only one port working, you need a router behind. But no point getting the Nokia ONR if you want to bridge the ONR now, since there is an option to go with ONT.
Singtel 500Mbps/1Gbps plan bridged ONR -- usually onely one port working, you need a router behind the bridged port.
Singtel 2Gbps/2.5Gbps/5Gbps/10Gbps) and WC 2.5Gbps, using ONR -- usually all ports will still work (one port bridged, the other ports not bridged), you need a router behind the bridged port.
2. ONT -- not a router, you need a router behind.
1) ONT for single network plans (500Mbps/1Gbps/5Gbps/10Gbps plan, and SIMBA 2.5Gbps plan) -- only one port working, you need a router behind usually. Whether you need to put the router in the DB box is another story. Typically yes you need. But then ther are other options, for example, if you have two LAN ports next to each other in the living room (Option 2 in the below FAQ).
Please read the following FAQ.
https://forums.hardwarezone.com.sg/...home-networking.6653421/page-3#post-149431568
2) ONT for dual 1Gbps plans (Starhub/M1/MR 2Gbps plan) -- two ports working, two independent networks, usually you need two routers to use the network. And usually one of them needs to be in the DB box, but you may be able to put the router out of the DB box. Please refer to the above FAQ. You can also use a router with Dual WAN Load Balancing feature for a single network setup.
3) Special configuration with Singtel ONT (only legacy 500Mbps/1Gbps users)
Singtel ONT --> VLAN Capable Switch --> two routers
https://forums.hardwarezone.com.sg/threads/working-singtel-vlan-settings-with-tplink-sg108e.5746952/
