yeah I had the same problem with my JXD S5110B. The WiFi antenna was stuck onto the battery, which I guess was effectively earthing it. It was an easy fix: open up the device and stick the antenna somewhere else. My WiFi was great after that
The same should be doable with the JXD S7800 although I haven't needed to open it up, from what I hear its slightly more difficult because of the ribbon cable, but it should definitely be doable.