Firewall / Security programs
A lot of antivirus or internet security software tries to control which programs can access the internet. Often, these programs err on the side of caution and by default many programs are not allowed access. The easiest thing to try is just turning your internet firewall software off, to see if that allows you to join games. If it does, you can turn it back on and follow these steps to safely allow Roblox to access the internet:
Find your firewall program's control panel. Often you can find this in the taskbar in the lower right hand corner of your desktop. Otherwise, it is probably in your Windows Control Panel.
Find the program permissions section.
Find Roblox and allow it unrestricted access to the internet.
For help with specific software, please consult your program's help manual or website.
If you have any antivirus/antimalware programs running in the background, there's a possibility that they are acting as a sort of overly cautious firewall that may be preventing some important communication between Roblox/Roblox Studio and our servers. If this is the case, you may need to update your program or add Roblox to your its whitelist. For assistance with this, please click here.
The Roblox client isn't bound to use a specific port for outgoing data, rather it scans to find a free port on your machine. Thus if your router is blocking UDP packets on ANY port, you may run into problems. Ports for the Roblox client are selected and assigned dynamically for each game session.
If you're running a router, look up the router's manual and see how to unblock them. If you're running a DSL modem that blocks ports, call your ISP and have them walk you through how to unblock them.
For more information on technical related matters, please see the following links: