@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...

  @Jamval Yes I believe V 2.8 is V 2.  Also I would suggest since you mention X20 to look at the RE600X, RE605X, or RE615X, not the RE450, (which is an AC extender).

  @jzchen I successfully connected the RE450 using each of the three methods, web browser, Tether app, and WPS button.  And yes I did have to individually set up the connection to the 2.4 and 5 Ghz bands.  Using the WPS button, you just go through the procedure twice.  My X20 router does not have a physical WPS button, but you can do it through the Deco app.

My problem isn't with getting it connected to my network.  The problem is not being able to access advanced settings (firmware update) through the web browser after it is set up.  It only allows access through the web browser for initial set up, but once I've established the connection, the web browser redirects to a page that says "something went wrong" and any future attempts go to a "page not found".  Wifi settings shows the 2.4 and 5Ghz as available connections if I give them specific names and I can connect to them and I can access them through the Tether app, but advanced settings are not available.

The documentation that came with the RE450 shows speeds up to 1300 Mbps on the 5Ghz channel and 450 Mbps on the 2.4Ghz channel.  MY IP service is 1 Gbps (1000 Mbps) which I have verified at the main router.  But the max speed I get when connected to the RE450 is no more than 200 Mbps and 20 Mbps respectively when the RE450 is right next to my main X20 router.  There's definitely something wrong with this.

The only thing I haven't tried is during initial set up to leave the naming of the 2.4 Ghz and 5 Ghz bands at the default name instead of me entering a name for each.  The default name is the name of my deco network.  If you do it this way you don't see any differentiation between the main router, or RE450 dual bands so you don't know which one you have connected to.  But, maybe it is possible then to access the RE450 settings through the web browser IP 192.168.0.254.  I don't think it's worth the bother, as everytime I have connected the RE450 it screws up connections to several of my other smart home devices - but get this, it only screws up the connection to my TP-link Tapo devices.  My Amazon Echo devices aren't effected.  

I thought if I maintained brand continuity (TP-Link, Deco, Tapo) I would have less problems, but my experience has been the opposite.  Especially with my TP-Link Tapo Doorbell camera.  When it comes time for a new equipment it won't be TP-Link.

  @Jamval I would not use a WiFi 5 extender with a WiFi 6 network.  Also don't forget about half of the spec bandwidth is going to go towards backhaul.  Since you didn't mention how many X20s you have nor backhaul (WiFi or Ethernet) for each, you'll have to do the math on how much speed you can expect...

It's a bummer there's no option to check for updates on the Tether app for that model.  Mine do have that option.  If you're able to return/exchange the unit for one of the ones I noted you may have improved performance, and hopefully don't have to find an alternative way to check for available updates.

  @Jamval  Just to clarify I do not think the webGUI is accessible after the initial setup on any of the REs that I have.  The solution is to download the latest firmware onto a PC, laptop, or Mac, connect Ethernet from said device directly to the Ethernet port on the RE, enter the IP of the RE into a browser, and check/update it from there.  I believe it only allows local access which is via Ethernet.  (I am sure I have had to do this at least once on the RE605X....