Solved! Lenovo Miix 510 unable to connect to the TV and router at the same time via WiFi, but only when the screen is mirrored not extended.

[EnDy_S]

When I use the TV as an extended 2nd screen this doesn't seem to be the case. Can anyone explain what's causing this?

 

[helpstar]

I think mirroring is a direct WIFI link between your TV and the Miix and therefore can´t be connected to the router as well. While extending the Desktop it´s streaming the content via the router to your TV.

 

[EnDy_S]

That sounds really strange, but not a bad idea.
Reading your comment I realised that there's another thing I forgot to mention. It's not that I absolutely cannot connect to the router, but frequently disconnects, and I cannot reconnect after that. So to be precise, there are small windows when I'm connected to the router while mirroring my screen.
This seems to make what you mentioned less likely.

 

[helpstar]

Is your router´s firmware up to date?

Is the wifi driver of the Miix up to date?

 

[EnDy_S]

The wifi driver is up to date, but the router's firmware isn't, I'll follow with an update later, thanks for the tip.