@Jamval Typically these get assigned an IP address by the primary router's DHCP server.  You'll need to log into the X20 management page and find what that IP address is, then you can enter that IP address into the address bar and log in.

Another possibility is to try the TP-Link Tether App.

  @jzchen I should have mentioned that I already tried both of your suggestions.

I opened the Deco app and found the IP address assigned to the RE450 and entered that in my web browser.

I tried this once while wifi connected to the main Deco x20 and again while wifi connected to the RE450.

Same result "page not found".

I also logged into my Deco x20 by web browser and found the same IP address assigned to the RE450,  Then tired that IP address in the web browser - "Page Not Found".

The Tether app is an abbreviated management interface with no ability to check or update firmware version.  

I also went into my PC's network settings (Windows 11) while connected by wifi to the RE450.  WIn 11 network settings shows the same IP address for the RE450 as was shown in the Deco app and web browser management.  Yes, it's different than the IP address used for initial set up, but it still doesn't work - "Page not found".

In the end, I suspect that even if am able to check and update firmware my download speeds will continue to be excrutiateingly slow when connectex to the RE450.

  @Jamval Is this set as a Repeater (wireless) or Access Point (wired)?  I know in the past I could connect an Ethernet cable to the Ethernet port and access the config pages locally.  Are you able to try that?

On the three repeaters I have the Tether App has an option to check for updates, but they seem to be newer models...

  @jzchen So far I've been only trying wifi connection.  The next thing I was going to try is what you're suggesting, connecting ethernet cable from my main deco x20 to the extender.  And setting operation mode to access point in the Tether app.  Just to see if I can get access to the extender in a web browser interface. 

  @Jamval It'll probably be easier to download the (correct) latest firmware based on HW Version # onto a PC or MAC, connecting Ethernet from the computer directly to the RE450.  There could be a number of reasons why there is no signal from the extender, one concern of mine is the instructions say to press the WPS button on both router and extender.  Then the next line says repeat the same for the other band.  I'm not sure if people are doing that?  I actually had to make sure there was a 6 GHz connection on my 6E extender...  The app does a good job showing the status of the connection of each band to the router, and also the broadcast repeat channel, so maybe I'm just being paranoid...