To get a new server list you need to navigate to the "pokerth" folder in your user profile.
You can find the pokerth folder by typing %appdata%\pokerth in the search box next to the start button and click open in the start menu. This will open the folder C:\Users\username\AppData\Roaming\pokerth
Navigate to ~/.pokerth or /home/username/.pokerth. If you don't see this folder you need to press Ctrl+H to show hidden folders in your file explorer.
This folder contains a cache folder. Delete either the whole cache folder or the 2 files in it.
The next time you start pokerth it will download the current server list and show you the list of servers when connecting.
There you chose the first server.
If this doesn't work, check your settings. It should be set like this