Virus through WiFi router/Modem?

Hi John; your wifi per say can not be infected with a virus; although it can be hacked into and gain access to devices connected to your wifi and having the connected devices infected with a virus; someone must dislike you pretty bad to target your wifi for such malicious attack. In my opinion; your mention of surfing some adult sites might be your source of virus infection. I will recommend you get a good antivirus for all your devices.

While Marks answer is a good one, and one I strongly recommend.

In this case your provider hijacked your connection.

It could be when you surf to any web site so they can send you notifications as an example your behind on your payments or to send you missing child or elderly person alerts "Amber Alert" here in the states. What they are doing is redirecting your http connection to their server to offer up the notice when you first connect to a web site.

Popup's can also be done, mostly this is frowned apon as it also implies your sessions could be monitored! Unless you are connecting to the providers web site first to gain access to the Internet this should not be happening.

I would speak with your provider to get clearification on why they are using pop ups. And if you can bypass it. I don't have a problem with notifications just in the way they are done. In this case it's on the edge of being an advertisement as they wanted you to call them.

The proper way of notification would have been sending you an email giving you more details of the risk (if there was one truly).

Never call unknown phone numbers on a land line. Only do it from a cell or VoIP connection and make sure to turn off caller ID so they don't get your number. This also holds true on any unsolicited emails or phone calls. There are just to many trolls out there who want to bang you for something and it may not be good!