The IPv6 support for the Guest WiFi feature was introduced in F-Secure SENSE Router firmware version 188.8.131.52. If you had the Guest network feature enabled for your visitors prior to this firmware version, the settings for the IPv6 support are not activated. To fix this, you need to turn the Guest network off and back on again in the following way:
Open your F-Secure SENSE Router app.
Tap More on the navigation pane at the bottom of the screen.
Tap Settings > Network.
Under Guest network, tap Off to turn the Guest network off.
Then, under Guest network, tap On to turn the Guest network on again.
From now on, IPv6 support for the Guest network is activated.