On Windows 10 as well as OS X v10.5.1 and later, sometimes the firewall will block trusted apps from accessing the internet. 

If this is the case you get the following notification when you open the RoomSketcher App:

Your request requires your device to be online. Please try again later. 

You can allow an app through the firewall, please follow the steps as described below for Windows and Mac. 

• To allow an app through the Windows 10 firewall
• To allow an app through the Mac firewall

To allow an app through the Windows 10 firewall

1. Open Windows Security.
   
   
   
   
2. Click Firewall & network protection. 
3. Click Allow an app through firewall.
   
   
   
   
4. Click Change settings on the top. 
5. In the list of Allowed apps and features, tick RoomSketcher. 
   
   
   Private: Allows the app access to the network at home or work.
   Public: Allows the app access to the network at a public place, such as on a coffee shop.
   
   Tip: If RoomSketcher is not in the list, click Allow another app... and find the RoomSketcher application (usually under C:\Program Files (x86)\Roomsketcher).
6. Confirm with OK. 
7. Open the RoomSketcher App and start drawing. 
   
   

To allow an app through the Mac firewall

1. Open System Preferences.
2. Click the Security or Security & Privacy icon.
3. Select the Firewall tab.
4. Click the lock icon in the preference pane, then enter an administrator name and password.
5. Click the Firewall Options button
6. Click the Add Application (+) button.
7. Select the RoomSketcher app.
8. Click Add.
9. Click OK.
10. Open the RoomSketcher App and start drawing. 

Note: The steps are taken from Apple's Support page where you also find more information.