Yes, it is possible to place the router in the computer room and connect the ONT to it through the PLC adapters, as PLC devices simply transmit Ethernet traffic and do not interfere with the network protocols used between the ONT and router. As long as your ISP does not require a direct physical connection for a specific setup, the router should establish the internet connection normally. However, keep in mind that the stability and speed of the connection will depend on the quality of your home's electrical wiring and the PLC adapters. Since your internet plan is 300 Mbps, using the PLC link between the ONT and router should generally work well, although a direct Ethernet cable would still provide the best reliability and performance.