We have some good suggestions for connection issues on our FAQ page here:
https://www.simplify3d.com/resources/faq/#quest-6354
For the Duet, make sure you are using port 80 and then enter the IP address for your printer. It should be able to connect after that.
We support both standalone and SBC mode for the Duet, so either should work. It's been quite a joy to use it with all the Duet printers in our office, so we hope it works well for you!