If you are playing Rainbow Six Siege (afterwards RSS) and experience issues like the one shown underneath this Guide might help you. In advance you should check your windows firewall und antivirus firewall settings for any restrictions assigned to RSS.

In this step we are going to open the Windows Command Shell.

Press "Win + R" and enter "cmd" without quotations.

To get your Router IP insert "tracert google.com" without quotations.
Normally your Router ist the first one shown in the list, the second one normally is your modem.

Open your Internet Browser. Type in the IP recieved in Step 3 - In my case it's 192.168.1.1- and press ENTER.

If prompted for Username and Password insert them accordingly to your Router Manual. If you have changed them you better remember them :)
If you lost it, a hard reset of the router can be done - Instructions in your router manual :)

The most used default combinations are (Format: USERNAME - PASSWORD):

Admin - admin
Admin - password
Admin - [LEAVE CLEAR!]
admin - admin
admin - password
admin - [LEAVE CLEAR!]

Afterward it should look a little like that (All manufacturers use diffrent GUI's):

This is different with every Router Modell.

Search the categories for UPnP.
Here it is NAT/QOS -> UPnP.

Often it is the same category (to see it you maybe have to go to advanced settings first) but can also be found under 'port triggering' or similiar sounding stuff.

Enable UPnP, apply and save the settings and you're done! This will enable automatic port forwarding on request of various games and applications. Note that you expose yourself to some risks by enabling UPnP. ( more[goo.gl] )

Alternativly you can enter those ports in the category 'Port Forwarding' to enable just the ports used by UPLAY and RSS.

Uplay PC:
TCP: 80, 443, 14000, 14008, 14020, 14021, 14022, 14023 and 14024.

Game Ports:
UDP: 3074 and 6015