DIY Power Bank For Router & Fiber ONU
If your current out of the LM317 is Iout you are dissipating 3V*IoutA power at the LM317 as heat. Also, check what are your current ratings of your router and your ONU to calculate your power requirements and check if your supply can handle it. Measure the voltage and current at the 5V supply output to see if it is able to support the load.