you can hook all washing machine to basically a single mobile connection, but preferably you're gonna want a hard line.
this! if you fronted up the cost of a hard line you could have wifi running and attempt to charge people to use it while they're waiting for their washing... not sure if that would be successful these days where almost everyone has cheap mobile data.
Even in my little semi-rural midwest college town, we have a laundrymat that has wifi available already.