ATM Network connection

We have a few ATM machines (where you can buy Cryptocurrencies) in our office and they are all connected to our office network inside our firewall. Now we want to place our ATM machines around the city and we don't know yet how they will get internet connection. We considered using 3G usb Dongles connected to each machine but I am not sure how secure it will be and how we will reach them for instance if we want to apply some updates .

Any thoughts about how to accomplish this would be very appreciated.

Thanks.