Laptop can't connect to internet while main computer is on?

[Munimentum]

Hi, so I've been having a strange issue I'm hoping people can help with. My main computer is connected via ethernet cable to my router. I have a laptop downstairs which connects wirelessly through a TP link wifi extender.

When the computer is off the laptop has internet access no problem, however as soon as the computer is on, the laptop has limited/no access to the internet.

Does anyone know what might be causing this?

Thanks!