@RaceTrak 

I rather suggest modifying your existing (AC1300) Deco mesh, assigning Wilma as SSID to your main network, and Fred as SSID to your guest network.

After that, you can add the 3 Deco BE5000's into the mesh, and adapt the mesh as you wish.

I tested connecting the Main Deco of one mesh (Jupiter) to a Deco of another mesh (Saturn), to obtain access to Internet.  I succeeded only when the operation mode of the Jupiter mesh is set to Access Point, which is, I believe, good enough for such a setup.

Hence, if that test is applicable to your models of Deco, you could do what you describe, "Wilma would have to access the internet through Fred".

  @yves_b 

Thanks for that thought -- I hadn't considered that approach.

I'm not sure that will work though because the Guest and IoT network names are based on the main network name and can't be changed (at least, not as far as I can figure out...).

That is, if I change my existing SSID to the new network name Wilma, I'll have Wilma_Guest and Wilma_IoT - I don't see a way to change "Wilma_Guest" to "Fred."

Thanks for the replay and also thanks for confirming that using the main deco of a new network to connect to the internet via different mesh network doesn't work...

  @RaceTrak 

By tapping on the name of the guest network, can you not modify that name?

I tested connecting the Main Deco of one mesh (Jupiter) to a Deco of another mesh (Saturn), to obtain access to Internet.  I succeeded only when the operation mode of the Jupiter mesh is set to Access Point, which is, I believe, good enough for such a setup.

Hence, if that test is applicable to your models of Deco, you could do what you describe, "Wilma would have to access the internet through Fred".

  @yves_b 

Wow, do I feel dumb -- I didn't even try tapping the name to see if I could modify it! It just "looked like" it was a fixed name... 

Thanks for the tip about changing the settings to access points. I will try that out!