ONT traffic is shared with you and the neighbours but it’s encrypted in transit. The OLT won’t be having any of your shenanigans unless it’s provioned at the ISP side. ( Very unlikely )

The ONT will assign one IP to the first device that connects. Every other device will look for a DHCP server to get an IP and the ONT will not allow them to get an IP.

You will have one device with and IP and everything else will be stuck.

This is why you have a router. A router has a front end to accept that one IP address and then use its own internal DHCP to allow everything else to access the internet, THROUGH the router. It is a one to many situation. A switch does not have this capability.