|For Teamviewer, nothing has to be forwarded in the routers setup. It works without intervention, because it establishes a connection to a teamviewer server in the internet and gets forwarded to the one, how has the correct login data.|
So if your friend has forwarded port 80 to your computer and your routers configuration interface also uses port 80, you can't connect.
The only way, to get around this is, to reset the router to it's factory default settings and reconfigure it from scratch.
To do so, read the manual of your router.
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