I have to reconnect wifi after download or my internet browser won't work.

If I start a large download it will work fine unless I try to browse the internet. If I do that then the download speed drops to zero and I can't get to any web pages. The only way to fix it is turn off wifi and reconnect. I have Kaspersky antivirus software and the setting on it have not been changed before or after this started. Resetting the router does not help this problem.