As long as you can connect to the data source(s) in question from where you are and you can import all the data you need into a Power BI Desktop project, you should be able to do all the work of designing queries, data modeling and report design without any need to access the client's Power BI portal.
Once you have completed your data modeling and report design work on the Power BI Desktop project, either you or someone who works for your client shoould be able to publish (i.e. upload) the PBIX project file into a group workspace. After that, the project's report is ready for use. There might be a need for configuration of data source credentials after ou publish the project, but that is pretty quick and would just take a few minutes. So, yes, you should be able to do all your data modeling and report design work without having to access the client's Power BI portal.
The one gotcha here is dashboards which cannot be created within a Power BI Desktop project. If your project requires dashboards in addition to the report provided by a Power BI Desktop project, then the dashboard must be created while accessing the client's Power BI portal.
