I resolved the issue by replacing the TL-PA1016 Kit with a kit from a different vendor. All other components stayed the same. I am now able to view IPTV in full HD on all three STBs.

@Peter: Thank you for your insight. Curiously my ISP has specifically stated to use a standard unmanaged router with a minimal of features. And honestly with everything cabled it all works perfectly....

Model : Hardware Version : Firmware Version : ISP : I have IPTV from my ISP The port on the router dedicated o IPTV is connected to a ethernet switch IPTV Box 1 is connected directly to the switch...