We use Sumups in my wife's business. One older machine that needs the phone app to connect, another one is Solo and can do that or uses it's own 4G to connect. I think they charge 1.69% fees.
On a busy market you can have signal problems if everyone's hammering the nearest mast, and occasionally the actual app will go down, but from speaking to other traders who use all of the available products: Square, iZettle etc. they're all about as reliable as each other. We have so many customers who pay with card or phone contactless and don't carry cash - especially at festivals or fairs. More regular markets or events in town centres ted to be a mix of cash and card.