In order for remote access to a FLEX-6000 to work using SmartLink, the Internet router where your radio is located must be able to accept an outside connection from the Internet in order to make a connection.
By default, a router/firewall blocks all incoming connections or "incoming traffic" unless they are explicitly allowed to do so. Allowing specific incoming traffic is done through a process called port forwarding which instructs the router/firewall to allow specific types of traffic, defined by "ports" to communicate with devices in your network. The traffic destined for a particular port is allowed to traverse the router/firewall and get forwarded to the intended device, which in this case is the FLEX-6000, hence the term "port forwarding"
Port forwarding can be done automatically when the FLEX-6000 is powered on using a feature that is standard in most modern routers/firewalls called UPnP (Universal Plug-n-Play). If your router has this capability and it is enabled, then there will not be any intervention you will need to do to your router to make SmartLink work.
In the rare circumstances where the Internet router does not support uPNP, you will have to manually set up port forwarding on your router. Unfortunately setting up port forwarding is not a service that is provided by FlexRadio support, so the following resources are provided to assist you if it is needed.
The following WikiHow article, How to Set Up Port Forwarding on a Router, covers the basics of how you modify a router for port forwarding.
A more in-depth setup description of port forwarding can be found at the portforward.com website.
There is even a section on the portforward.com website that lists instructions for setting up port forwarding for specific routers and their list is very extensive.
For determining exactly what port numbers and type you need to configure for the FLEX-6000, please refer to the SmartSDR v2 Software User's Guide for instructions on how to configure manual port forwarding for SmartLink.