You’ve probably reached this page in a state of frustration, so I’ll post the answer first and then the explanation. In this instance, I’m talking about TP-Link devices. I’ll refer to the Extender and the Primary Router.
Factory reset your Extender.
Connect to your Extender, either over its default WIFI network or with an Ethernet cable.
Assign a static IP address to your Extender in the same IP range used by your Primary Router. The Default Gateway address should be the IP address of the Primary Router. The IP address should be outside any DHCP ranges you use on either device.
Configure DHCP on your Extender exactly the same as on your Primary Router (I know you’re advised not to do this, but bear with me.)
Do not make a WIFI connection between your Extender and your Primary Router (i.e. don’t follow any quick setup wizards. If possible, disable this capability entirely.)
Set your SSID to anything you want, but make it’s something different than the SSID of your Primary Router.
Restart your Extender (at the power switch if you want, but give it a couple of seconds while off).
Next, on your Primary Router, look for the “Enable WDS Bridging” option, which you will probably find as a Wireless setting. Configure this to connect to the WIFI network of your Extender.
Reboot your Primary Router.
Finally, on all your devices, stop and start the WIFI connection.
You should now be able to connect to either network (SSID) without IP conflicts.
Note: your Extender will probably maintain a warning led, but this won’t impact on your connectivity.
Consumer WIFI Range Extenders are generally designed to extend a WIFI network into “dark” corners of your house where there are static devices (e.g. PC in kids attic bedroom). There are not designed to permit roaming around your house with a mobile device like a phone or laptop. Unfortunately, manufacturers don’t tell you this.
When you connect your Extender to your Primary Router as instructed in the documentation, you are effectively turning your Extender into a router. If you connect your mobile device to the Extender and the Primary Router at different times, both devices will have a copy of the MAC address of your mobile device in their routing/bridging tables. When either the Extender or the Primary Router recognises this, it will broadcast a conflict on the network which will appear as an error on your mobile device.
When you instead use WDS Bridging Mode, you are turning your Extender into an Access Point (AP), which means it never stores your MAC address, and instead just forwards all packets to the Primary Router (which does store your MAC address). That means you can connect to either SSID but your MAC address will only ever appear in a single place (so no conflicts).
You need to ensure DHCP is enabled on your Extender so that devices that connect to it get a valid IP address. I’m not 100% sure if you need to specify the exact same available range on both devices. It’s probably OK to split them. The same range worked for me but I have only a small number of devices. Make sure you use the same Default Gateway on both.
Not sure about other manufacturers, but the documentation available for TP-Link devices is beyond dreadful. It all sounds like it was written 10 minutes before the products were shipped and not actually tested on real users.
Also (and this applies to nearly all technology), if anyone on a web forum tells you to update your firmware to resolve issues like this, please ignore them (particularly if they work for the manufacturer).
Firmware updates are generally about Linux kernels updates and security fixes for libraries like OpenSSL. They almost never have any influence on the basic functionality of a network device.