a) Foris → DNS → Test connection: If that fails, select a DNS Forwarder with ‘TLS’ in its name.
This makes sure, DNS is not seen by your Internet Service Provider (ISP).
b) Foris → WAN → Test connection: If that fails for IPv6, disable IPv6 on that page.
c) If the last test failed but you have no WAN just LAN in Foris → Network Interfaces, disable IPv6 on your primary router (cable modem, DSL modem, fiber-optic modem, …).
If none of those alternatives helped, please, continue there… sometimes it is enough to do the DNS test and then go for Foris → Update → (button) Save and update, because DNS got cached by the test.