@Ouremcee 

Hi, Thanks for the feedback.

I think your issue might be like this:

https://community.tp-link.com/en/home/forum/topic/616724

Can I have the model numbers of the two network switches?

Did the network topology remain unchanged before and after the firmware upgrade?

Wait for your reply.

Best regards.

  @Ouremcee 

Hi, Thanks for the update.

I see NETGEAR GS308 is an unmanaged switch, which is almost the same as TL-SG105.

But I happen to see there is a similar model NETGEAR GS308E which is an easy smart switch and supports VLAN Configuration.

Could you please confirm whether the switch is NETGEAR GS308 or NETGEAR GS308E?

If it is the second one with "E", I think there still needs addtional VLAN Configurations on the NETGEAR GS308E to make the guest network work properly on the third satellite Deco M9 plus:How to extend guest network when there is a switch between Decos

Or Could you please help me run a simple test:

switch the location of these two satellite M9 plus units to see whether the issue follows the same unit or the same switch.

Wait for your reply.

Best regards.

  @Ouremcee 

Thanks for the update.

I will wait for your good news and at the same time, I would also like to forward your case to the senior engineers for further assistance.

Please check whether you have received an email from us.

Thank you very much.

Best regards.