Issues with mobile phone & wifi

I have a strange problem with my wifi at home.

I have 3 devices which connect to the router, a laptop, a notebook and a mobile phone.

This past week I've found that the phone won't connect unless the notebook is also connected (the laptop is almost always online).

I can't work out why, but unfortunately the router is one of those which is custom set by the ISP and I can't re-configure it. FWIW it's an Arris.

Thanks in advance for any suggestions