Hi. You are on the right path with power line plugs but you need to buy power line that also has wi-fi .... but I would highly recommend you use a good name brand and not the Johnny washable off eBay
I used to use netgear a lot but have moved over to TP link ..... more reliable and I would say best on the market to buy
type to buy ... depends on your budget just look for T-P link with the part number starting with WPA as this means it also has Wi-Fi
this one will give you full Wi-Fi in your 200ft garden
WPA8630 KIT
but you can buy TP link cheaper models