If you just want isolated guest wifi with bandwidth limiting, you can do that via the Unifi controller with a guest SSID rather than setting up VLAN’s and port tagging. Obviously it depends on what your end goal is and if you need more granularity over routing/filtering than this.
I thought a USG was needed for that?
What Draytek router have you got? Is it VLAN aware?
Essentially, create the VLAN in the Draytek then create it in the UniFi controller (as a VLAN only, not a Corporate network) making sure that the VLAN ID's match. Make sure that the switch port that connects to the AP's and Draytek is tagged in that VLAN then create your new WiFi network and give it the VLAN ID you selected earlier.