Sounds like a matter for the union.
Why? In the first instance is a reasonable conversation with a manager or HR representative not the first port of call, if you truly feel it is unreasonable. Maybe it’s just because I don’t come from an industry that is particularly unionised (advertising and media, ironically), but I don’t see why an issue like this is one for the union at this stage, when it can be dealt with yourself.
For what it’s worth I think their change is reasonable, frustrating yes, but reasonable. I think it’s only reasonable for them to foot the bill for things where there is an additional cost to you having them to do your job, these days broadband is not one of those things as it is reasonable to expect people will have this in their home anyway, just like I wouldn’t expect them to contribute towards a chair or table for her to sit at.