You are both using a broadband modem that connects to your computer via USB, a system that was never designed for networking, uses a huge amount of computer resource and is prone to break down. The only reason that ISP's gave these things away is because they were cheap (and nasty).
If you want something that works properly, get a router that connects to your PC via ethernet, the system that was actually designed for networking.
Have a look at this for an example of what to get.
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Do you know that we're all in line for succession to the throne? Really?
Well, if forty-eight million, two hundred thousand, seven hundred and one people died I'd be Queen.