So a housemate in this current houseshare I am living in decided to change the password to the WiFi, which is owned by the landlord and is included in the rent. I've already informed the property owner but the housekeeper is not really sure about what to do other than removing the router from his room. I have a bad feeling this could happen again even if they somehow resolve the issue this time.

Is it actually legal for him to do something like that? I have evidence that the router information has changed and was not by the hands of the landlord. Also "talking it out" is not really an option, he has threatened me multiple times in the past and I have been advised to "avoid interactions" with him.