I have a 3 foot table, with occasional connection "quirks". I'm trying to identify the source of the issue, both to make my family happier and to help others do the same.
I think that part of my problem is that the antenna is on the RPi board itself. I'm reasonably sure this is an omni-directional chip antenna, but it is going to be influenced by the structure of the table, and especially by any metal parts. It likely does not help that the direct line of sight from my table's RPi to the nearest access point passes directly through the metal band around the rim of the table. (The part that supports the outside veneer band.)
I know the top most veneer ring is glued to plexiglass, and unless someone decided to aluminize it, that, the wood, and the glue would be nearly transparent to 2.4 GHz.
The RPi is attached to the underside of the plate that holds the sisbot's motors and the rest of the electronics. I suspect from photographs of the sisbot removed from the table that plate is aluminum. If so, that would likely be a reflector at 2.4 GHz, and limit the field of view of the antenna by pointing the signal generally towards the floor.
What is the sand tray made of? I know it can't be anything ferromagnetic or the whole thing wouldn't work at all.
My WiFi is provided by a collection of Ubiquity Unifi AP and AP-AC-Lite access points. I can always add one more if I can find the right place to put it...