@MichaelBlucher This is what I know from fiddling with the RE450 for the last few days. On some routers, it works just fine on its default easy setup. On others, like my old main ASUS router, it will work briefly and then no longer provide ip addresses to wireless clients. After some research, there is a compatibility issue due to the lack of standards for certain wifi configurations and different brand equipment is not always interoperable. But there is a fix

To fix this, I followed TPLink's instructions at the beginning of this forum and it seems to work well

You must set a static IP address for the RE450.

I use a static ip address that is outside my main routers DHCP range (192.168.1.1 to 192.168.1.100). I set the RE450 to a static IP of 192.168.1.117

After that, I turn on the RE450's DHCP server and it it's range of addresses separate from those of my main router. In my case 192.168.1.150 to 192.168.1.200

Most network experts will tell you that it is advisable to only have one DHCP server on a subnet. However, two or more will work reliably as long as you keep their ip address pools separate. This is what I have done from TP Link's instructions and it seems to work well with the RE450 providing IP addresses to the clients when the main router fails to do so due to the brand compatibility I talked about.

If you want to research the reasons for this lack of interoperability, look up WDS or Wireless Distribution System. It was not standardized well.

@kyanos 
no need.

normally, if the re450 was set up successfully, the DHCP is supposed to be off.
this option is just used when the DHCP was not off after the setup  but all the lights on the extender lit up, we might need to set up it manually to be the same as the main router.

for your case, it would much better to reach for the tplink suppost team to get more advice. they are excellent!
https://www.tp-link.com/support/contact-technical-support/
thank you~

@Lawrence-C If you set the RE450 DHCP server to the same pool of addresses as the main router, there is the possibility of conflict. I know that DHCP servers are supposed to send out an ACK to check before assigning a new address, so most of the time this will work. However it doesn't always work with some routers and I have come across situations where the same IP address has been assigned to two different devices, resulting in one device losing connection. It's safer to segregate the pools. I've been monitoring my network set up this way and its quite interesting. Most of the time, when I connect to the RE450, I will get an IP address from the main router. However occasionally, the RE450 will assign from its own pool. Everthing has been stable for about a week.