Christian, try the steps in this post - if you do not get this:
then your provider might not give your connection a public ipv4 address (Carrier Grade NAT GNAT), and you will have to ask them to change it for you.
EDIT: Just noticed that the Roon error message shows the external ip as router_external 192.168.100.3 which is a private IP, which means that probably your linksys router is seen as the external router and not the ISP provided one.