Officially the EU requires allowing customers to use their own modem if they want to. If the ISP setup requires registering MAC addresses they are obligated to register your personal modem and if a weird setup is required they have to document everything and notify you in advance of changes.

Do you have any more details about this? A link or what to search for would be also great.

The directive is 2008/63/EG.

https://eur-lex.europa.eu/legal-content/EN/TXT/?uri=celex:32...

Can only see one example of someone asking about that on google.

I do not understand what you mean by that. Note that these directives force the member states to adopt their own laws implementing them. They do not directly affect people, only indirectly. So if people want to invoke the rights they have to cite the national law implementing the directive.

The response in the UK is always "you can't do that", so either the UK hasn't implemented the law, or no one cares

The directive stipulates some authority who enforces the rules.

It’s quite possible that, similar to the situation in the Netherlands, the rules don’t align with the opinion of the appointed authority and any appeal is nullified by an endless stream of denials and delays.

For a truly determined citizen or group it would be possible to work through the national and European legal system and force the authority to do its job but it takes a very long time and a lot of money and determination.