I found this on Phoenix's website ill look into it.
When attempting to join an online session, I cannot connect to the session or see all pilots
Phoenix uses a complicated system including a dedicated "master" server in order to make connecting as easy as possible, even if both the session creator and joiner are both behind routers and/or firewalls.
Despite these extra measures, some routers and firewalls will not allow this process to happen. This is partly due to the protection this gives your computer against malicious attacks from the internet.
In these cases, you may need to "open" or "forward" the internet ports on your router that Phoenix uses for online flight to your computer's own address.
The ports that Phoenix uses are UDP PORTS 26500 - 26505.
There is a great deal more information on how to accomplish this on the internet, on website such as PortForward.
If this fails to resolve the problem, you may consider changing your router to a more compatible make and model.