Check the circuit environment as this FAQ: http://www.tp-link.com/en/faq-409.html And if the link speed between the PLC is slow, maybe you have some devices have filter function

you can ping the AP successfully when you drop connection? Does it appear limited Internet access? maybe you should check the connection between router and AP

If you want to extend wireless signal---No If you set RE450 as an AP----Yes

Could you log in the web management of the router via connect the WiFi from RE450? Have you try scan the wireless signal manually? Does it work?

log in the web management to change these setting

wireless signal is easily influenced by wireless environment and PLC will be influenced by circuit environment. The wireless environment may cause the buffer(wall, mirror, metal and microwave oven,.....