To have just one VLAN or SSID affected by the user login, you need to make filter rules on the router that cover the IP ranges of the other networks - that makes them exempt from the user login requirement.

To do that, you'd make a rule under Firewall - Filter Setup - #2 Default Data Filter, rule 2 (since it's unused by default) and set it up with these settings:

Direction: LAN to WAN
Source IP: Edit that and enter your main subnet as a range address or subnet address
Destination IP: Any
Action: Pass if no further match (just in case you do want to firewall stuff in future)

That will then exempt that subnet from user management so they'll be able to access the internet as normal, you can make rules for other subnets if you've got multiple subnets in addition to the guest network IP range.

Update to 3.3.7 and guest portal options are now available