Removing root is more than uninstalling Superuser. Superuser is just an app that keeps a list of apps you've told it can access root when they ask for it.
Try
[Samsung] How to flash Stock ROM via ODIN, Samsungs are pretty easy to flash. (After Odin finishes it's going to take 10-15 minutes of watching grass grow, so do it when there's a good TV show to watch.)
However, rooting and TWRP shouldn't affect Wifi - all of my devices have both (as an old hand at Linux [which is the OS on an Android phone] I just can't run without su [which is root], and I find TWRP handy - like for backing up before an update, in case something goes wrong [which happened twice already on my 2 month-old phone]). I suspect that the tablet has a problem, and your best bet, if you can (or contact eBay if you can't) is to return it for a refund.