WiFi keeps dropping in the back office - small cafe

We run a small cafe in central Manchester and the WiFi keeps dropping in the back office. The router is up by the till, and from the kitchen and the upstairs office the signal cuts out two or three times a day. The card machine works fine but the laptops we use for orders lose connection mid-ticket which is a nightmare when the queue is out the door. We've tried moving the router and a friend gave us a cheap booster from Amazon but it didn't help. Looking for someone who knows commercial setups, would rather it just worked and didn't need us to keep restarting things. Cafe is open 8 to 4 weekdays so any visit would need to be before opening or on a Monday when we're closed.