Tenda TEG1024D - Switch - 24 x 10/100/1000

So if i already own a modem, can i move all the units away from it after initial setup, or does one have to replace my modem?

If I currently have a cable modem and wireless router in one, do I need to purchase a cable modem?

Signal strength is showing excellent on each node, but I get better download speed if I disconnect them and just use the gateway, why?

Im in an 1100sq ft apartment on the 2nd floor. If i get these, will they be able to reach outside to the yard to use my firestick in the garage?