There is a bug where if you change regions while connecting, the game may overwrite your region data with the wrong region. This makes you connect to the wrong region later. It can easily cause room codes to not be found. You can fix this by deleting the serverInfo2 file found at%appdata%\..\LocalLow\Innersloth\Among Us
Then restart Among Us.
On mobile, you must find this file in the appdata or clear the game's local data. (Clearing game data is not the same as reinstalling, but that might work as well)
If you need help with this, you can find me on discord [Baron#8083]