So with the new firmware update the guest network now works with the GT-AC5300 but it does not works with the other 2 routers (RT-AC88U and RT-AC86U)

Thats good news, my ax11000 firmware was outdated. I have just updated and at least in the living room where I have a GT-AC5300 everything seems fine now. I will soon be testing with the RT-AC86U and 88U that are located in different rooms. And if I ...

So the only solution found so far for this issue is to not use the Ai Mesh network? The solution would be to use all routers as APs? Sad..