Between 150617 and 170803, is there any firmware available? If yes, try to update the middle firmware, then try to update the latest FW 170803.

hey, you can go to Advanced--Security--Access Control ..

How about the wired connection through M5? Does the main Deco still work properly? or all the deco units drop frequently?

Sometimes, you need to consider three address format or four addres format when you set WDS bridge connection. You can try to set TL-WDR4300 as the main router and set Archer C60 as the WDS repeater.

If you want to limit kids network activities, why don't you try parental control?

don't think it can be work as a LAN port even in AP mode. try to add another wireless powerline extender with three ethernet ports. then, you can add another two ethernet ports....