@FlyingBarney 

Is your X20 in guest cottage connected to your house network wirelessly, or by Ethernet cable?

If it is connected wirelessly, the only option you have is to enable Guest Network in Deco app. Make sure Guest Network has password (by default, it does not). Also, check if you have "Isolated From Main Network" toggle for Guest Network in WiFi Settings screen of Deco app, and set it to enable if you do. 

Provide your guests with Guest Network SSID only. Make that SSID completely different from Main Network, to avoid someone guessing Main Network SSID from Guest Network SSID.

To somewhat speed limit Guest Network, enable only 2.4GHz on it.

If your X20 in guest cottage connected to house network by Ethernet, I can offer you more options.

@Alexandre. Thank you. Followed your thoughts and was able to figure it out (wireless connection), finding out the guest network and changing its name/password from the main one, and also setting it to be only 2.4G. So this is exactly what I was looking for.

Appreciate your thoughts and promptness.