How to allow an organisation access my network to configure their equipment

You are an IT Security administrator in a banking organization. Your organization hired an outside IT firm to do a proof of Concept for new equipment which is a computer based system. This equipment is bulky and after importing it from Europe, it’s delivered direct to organization’s premises, for easier access and configuration to the client’s software and hardware platforms.
The hired firm starts off with their work of configuring the equipment but at some stage, they get a problem and need to connect directly to the equipment’s software developers who are in United States, so that they can access the equipment and be able to troubleshoot and install the equipment themselves in an efficient and effective way.
The contracted IT firm requests you (IT Security administrator) to connect to the organizations network so that the developer can carry out the installations.
According to your role, please advice the IT firm on the way to go by giving a clear explanation and a practical demonstration on how to go about this scenario.