help random devices showing up

So I’m just trying to understand why this may be occurring.... when I use Fing these devices named “Generic” will appear, and every so often they will change to new IP address. But when I check my router device list online (look at third picture), the devices I know of—will appear, but these “Generic” devices there is no sign of them whatsoever on my online device list .. Like if they are they hidden? What are they? Why are they constantly changing IP address? How can I fix this? Please Help with some knowledge