Connectivity Issues With Certain Devices

I bought this router last week because it is supposed to be so easy to set up and use. 

I have seen nowhere that if you have an existing modem router, without any options it just changes the ip of the Orbi. That then stuffed with all my Alexas and Lifx lights.

I finally found an old modem and, after factory defaulting Orbi, I was able to get back to 192.168.1.1.

I now have a somewhat functioning network at home but I would really like to get some answers to the following questions.

1 How can I stop the 5g network? I think it is the reason why my lifx bulbs don't work. I can find them on reset but when the app switches back the network it can no longer find the bulb and so the tethering fails. I assume that because I am using galaxy S7 edge,  I connect to 5g and the lifx only uses 2.4g.

Please help on this.

2 I have 2 surface 3 notebooks and initially they connected to the orbi without trouble but recently, first one, then 3 days later the other no longer have internet access on my network. Connect to wifi ok but no internet. when I switch to gurest network all is ok. What could the reason for that be?

Please help with this as well. I only have until Thursday to take the orbi back for a refund.

kind regards

Martin Brinkmann