@TP-Link 

Thank you, but can you please be more specific? I do have a guest network enabled, but how does that impact?

In the DHCP, where would I put the PI ip? In the DNS setting?

I also have a Pi as my network DNS server. (My Deco M9 Plus is attached to an ATT Arris BGW210 router). Sometimes my Deco accepted the Pi on the LAN, and then other times it balked and changed the network address (192.168.10.x, 192.168.68.x). I finally got it sorted out. What I've done is:

• Set the Pi as DHCP server and DNS server.
• Give the Pi a static address (it won't assign itself one via DHCP. I chose192.168.0.2).
• DIsable the DHCP (and wifi) in the BGW210.
• Make the Deco mode AP, not router
• Have the Pi DHCP use the range 192.168.0.100 - 192.168.0.200, gateway 192.168.0.254. When I first set it up, I chose a very short DNS lease (1 hour), after things looked stable I changed that to 24 hours.

My network now uses 192.168.0.0 - 192.168.0.255. The BGW210 is x.x.x.254, the Decos are x.x.x.1 and x.x.x.3, the Pi is x.x.x.2. (I used static settings in the Pi to assign the Decos and several other devices that I SSH into. Other Pi's are at x.6, x.12. In all, I have 12 static assignments in the Pi DHCP settings).