Warframe uses UDP ports 4960 and 4965 (or whichever ports are set at the in-game options) in order to let you join games. If you have more than one PC/Console playing Warframe these ports might conflict and the second PC/Console may be unable to communicate
To fix this issue, change the network ports used by the second PC/Console.
- Open the Main Menu (ESC on PC), select OPTIONS, then SYSTEM.
- Change the Network Ports [UDP] to another range of numbers (first PC/Console will be using 4950 & 4955 so pick another pair)