Get rid of the free WIFI.
Make sure the building is a great Faraday cage.
That's a tough balance to wire walk. It's an expected modern amenity these days and now it seems a penalty to service. It also impacts seat and table turnover which is important to the restaurant's bottom line. That still has no effect on the selfies and plate photography delaying eating the food which allows the food to cool.
A tastefully lettered sign saying "Please, Chef ___ requests that you enjoy the Food and the Ambience, not your Cellphone, during your meal at Chez _____. Thank you." at each table may work better.
I would never go there then ... :-) ...
Get rid of the WIFI and buy a signal jammer.
Ban photo taking in the restaurant for the safety of
“the children”.