Ensure the 5210G is connected to the home plug extension by wired.

Try:
1. Reset the WA5210G.
2. log in WA5210G, don't use Quick Setup, navigate to Basic Settings -> Operation Mode to select AP mode, navigate to Basic Settings -> Wireless Mode to select Access Point mode.
3. Navigate to Basic Settings -> Wireless to configure the SSID, channel, power. Try channel 1, 6, 11.

Note that the WA5210G is a outdoor directional ap, it focuses the signal to only one direction. So you need to ensure it is facing you, otherwise the performance can be bad. Also, WA5210G is a old 802.11b/g products, while smartphones nowadays prefer 802.11n mode. The phone may be incompatible with 802.11b/g, causing problems.

I suggest you to replace it with another model, such as TL-WA801ND, TL-WA701ND, this kind of AP is 802.11b/g/n compatible and omni-directional for indoor use.

Thank-You for your input, friends.

I have tried changing channels, but still the issue persists. Is it cause of the distance? The area where I am using the wifi is approximately 20mts apart from the access point.

Also, I am using the iPhone 5 and it says 'unable to connect to *SSID*'