My guess as the Laptop works next door, it is a compatibility issue between the laptop and the ISP router
Did plusnet provide the router? if so you could try replacing it with an off the shelf one. All the configuration deltails you need will be on the admin page of your existing one.
you can run "Task Manager" check for any processes or services you don't reconise and google search them.
This may help all being it is not your model but similar symptoms
https://forums.lenovo.com/t5/Lenovo-Yoga-Series-Notebooks/Yoga-7i-Download-Speed-Slow-Issue/m-p/5074667
Just out of interest what is the model of the laptop, also does it have a bandwidth restrictor installed something like "Rivet Dynamic Bandwidth Management"?
Not going to be much help, but my husband had the same issue with his Iphone 6, connected to our home wifi really slow, connected to office wifi everything fine. All other devices at home fine. Obviulsy something we can not check wth a phone was what it was liked when physically connected to the...