Hello @aj_p 

After RE connects to router's successfully router works an DHCP server and assign IP address all devices, so device should get an IP address which is in router's IP range/subnet. 

Does the 5Ghz LED light up on RE450?  If not, please factory reset extender try to reconfigure RE to Tether APP/ Web page.

• Before set up RE again, it is recommended verify the (5Ghz) Wi-Fi Password:

Go to Settings-> WiFi on your phone, Tap (or Long press  the router's Wi-Fi name  →Forget / Delete the network 
Please reconnect to your router Wi-Fi it will ask for a password, will it accept router password?

• For router's 5Ghz Wi-Fi channel,it is recommend to set on Channel 36/40/44/38 which are more stable. 

Hello@aj_p Thank you for your explanation, Mac self-assigned IP address xxx.xxx.xxx.xx means it does not get an valid IP address from extender, 

1. Please check advanced settings on router, is there any Parental Control/ Access Control ( Blacklist) settings that may block the extender

2. Connect extender to a different router or AP, or connect extender to a Hotspot Wi-Fi, and see if devices could obtain IP address from extender. 

By the way, extender will broadcast both 2.4G and 5Ghz even link to 2.4Ghz Wi-Fi

May I know how long did extender before, may I double-check the model of router beginning with Archer XX .

Do all the device connected to 5Ghz but has no internet / could not get IP from extender's 5G?