Would it not have occurred to you, or anyone else here, to take it back to WiM the moment it started running hot?? I would have had it back to them for a checkover the same day.
This is basic mechanics - if you rebuild a trans you check and clean through
everything attached to it - trans cooler, pipes, filters - the lot
before it is run. Check the pipes running to the cooler when the trans is hot; are they both hot or not? If not, hey, it looks like your cooler isn't working in which case its probably blocked with remnants of your old trans internals.
Did you change the OBD sensor after the rebuild or are you relying on the old one?
Driving it to try to determine why the trans is overheating has the same relevance as kicking tyres when looking at a new car
It might tell you its hot but not why, and its the "why" you need to find out.
It will run hotter than normal after a rebuild but only by a few degrees.