Unfortunately that leaves you in a jam. If management does not have policies or written guidelines your staff is free to make up the rules. Currently the rules include support for the WFH group by the in office staff. I would find a way to highlight the issue.

This should be addressed as a company policy issue. What policy does your company have governing working from home? Did your company outline how work will be performed from home? I would discuss how to handle this with HR since working from home is not available to everyone. At a minimum, everyone working from home must have access to all documents and resources required to successfully work from home. If this is not the case then either the company needs to accommodate them if the company allows working from home or working from home is not available and Personal Time Off should be used to cover the days not spent working successfully from home. If there are childcare or eldercare issues occurring due to the pandemic, the company needs to make their stand very clear in the form of written policy. No company can survive making individual exceptions as a rule. The accommodations need to be well thought out and fair and communicated in writing. The staff need to be told that support to personnel outside the office is not permitted unless approved. Scanners & printers should be provided to the work from home folks so they are not disrupting those in the office. Again, all of this is the responsibility of management to accommodate or not as part of their guidelines.