Internet upgrade to dual band without NID?

I want to upgrade my internet with dual band wiring. I don't have access to the network interface device for my house. The service comes in from underground straight into the wall cavity of the house and comes out at a contractor-installed box that distributes wiring throughout the house. The service line cable and the cable feeding into the box are different, so there must be a connector between them somewhere in the wall cavity, but it's not accessible. My question is, can the ATT field technician splice the service line where it comes into the house, or splice the other cable feeding into the box, to complete the upgrade to dual band wiring?