o if you feed the POU tank from the hot water line, where is the cold water that's in the line going to go? If you only need a little hot water, it will be fine, but once the POU tank is empty, you'll have cold water again until the hot water arrives from the main tank, because the POU heater won't heat the water quickly. Might not be an issue for a tub, but not what you want for a shower.
The cold water line IN on the POU water heater connects to the existing Hot water line. The output connects to the hot water faucet. The 2 or 3 gallons of water in the POU tank should be enough until the existing hot water arrives.
Retired Guy- Central Iowa.HVAC/R , Cloudray Galvo Fiber , -Windows 10