@schmieg 

Hi, welcome to the community.

You are right that the Guest network of Archer AX3200 does not support Onemesh. So if you plan to use an extender(for example RE815x) to expand the wireless coverage of AX3200, It needs to be set up in an ordinary way without OneMesh but the extended network of re815X would still be separated from the main network of AX3200.

Since the Onemesh feature is no longer used, the model does not have to be RE815X, you could randomly pick up another WiFi 6 range extender here:

https://www.tp-link.com/us/home-networking/range-extender/?filterby=5730

Thank you very much and best regards.

  @David-TP 

Would it be possible to retain the One Mesh for the main network and add another range extender for the guest network that would be set up in the normal manner?

  @schmieg 

Hi, yes.

  @David-TP 

Thank you. I thought that is how it should work, but I've run into strange things before. Now, I need to decide whether to expand the current set up or switch to a WiFi6e system for a lot more money.