Please note that an ISO 27001 ISMS scope is defined in terms of information, processes, or locations to be protected.
Considering that, you can try to define the customer scope in terms of the locations in the building it occupies (e.g., which floors or offices). Regarding the network, you need to identify how the IP range used by the customer is separated from other IP ranges you provide (e.g., by using VPN, by having its own device, etc.).
Considering that, if you are a small organization, it is best to define the whole organization as part of the ISMS scope (so in terms of Organizational Units you can state that all organization is part of the scope). If you include only one part of your organization, then under the 'Organizational Units' you list only your departments that will be included in the scope.
Regarding processes and services, these would be related to the information you want to protect. For example, if you want to protect customer financial data, then the financial processes and services would be included in the scope. In case it is the software you provide to the customer, then you should consider the development and operation processes related to the software (ISO 27001 cannot be used to certify products and services).