I am using a Balance 305 and just updated to firmware 7.0. My Guest WiFi uses a VLAN and has lost all internet connectivity. Upon further investigation it seems that DNS lookup is being blocked. This is only the case on the VLAN and not on the untagged lan.
I have triple checked the DNS settings and cannot identify the actual cause except that outgoing DNS traffic is likely to be blocked. I deleted all the firewall rules to ensure that these were not the cause even though it was working before the update.
I can ping the internet without issue and have created a few manual DNS entries in the HOSTS file which work as expected.