@chrisnz
Hello, being of two distinct networks which, I think, should not be able to communicate with each other, the solution is to add an interface to the pfSense router, in your case not physical.
Since your switch is web managed the best thing you can do is to create a VLAN dedicated to the Guest network and use the switch for all your private connectivity. And only for those!
You will find everything you need in the pfSense and Netgear documentation, in the respective sections that talk about VLANs.
Googling I found this which looks a lot like the recommended solution:
pfSense router-on-a-stick VLAN configuration with a Netgear GS108E
I hope it will be useful to you.