Bypassing RE200, your PC will always obtain the fixed IP address from the router? Provide firmware version of RE200 and WDR4300.

Are you talking about powerline extender? What's the model number pls? TP-LINK powerline devices can work with other brands as well.

Are there any other PCs with gigabit port that you can connect to LAN port of C5 and check the link speed? Did you try firmware upgrade or reset the C5? If all the same, the router should be...

Let's see if the replacement will solve the problem.

It should be a pair of MC112CS and MC111CS.

Sorry but I didn't get what you are trying to say... The router restarts by itself and loses configuration? How often does it happen? Did you hard reset the router? Is it brand new?