@Lincoln 

I realize the tp-link insructions recommend using a wired network, but I did my upgrade through the wireless network.  Were you wired through a switch or directly to the M9 Plus?  I read somewhere that the DECO does not play well with DLINK switches and had to pull mine out of the network when the LAN to my TV kept dropping out. 

@FA44 I connected the cable directly to the Deco M9 Plus

@TP-Link_Deco I'm using Windows 10 Pro Ver. 2004. The most interesting thing is that the failure happens in the login of the application, I'm using the same email and password that I use to enter the App for Android Deco.

@Lincoln  I have the same issue.  Tells me my credentials are incorrect but they work on the app and on this site. 

This whole thing is irritating.  Tech support has been useless and didnt even tell me about the M9 update tool, I had to find out here after they instructed me to use the WebUI which isnt functioning for our model.  

@itismeTravis 

Just a thought, check that "Fast Roaming is DISABLED" when you update (see note #5 below).  Not sure if this is the problem, but worth a look.
I agree this was very irritating because there was no clear way to find theTP-Link  instructions (https://www.tp-link.com/us/support/download/deco-m9-plus/v2/#Utility) on how to update my system from the Sept 2019 firmware as was reported in my App.  Going through the Deco M9+ App, it always reported firmware was up to date eventhough it was clearly a 2019 firmware.  Through this thread and a few other drill downs, I was able to find the utility/firmware and update to 1.4.0 Build 20200324 Rel. 53529 as noted in my post #20 about a week ago.  Once I made the March 2020 update, the App handles any new updates and has since updated all my 3 Deco M9+ units to the latest firmware version 1.4.2 Build 20200725 Rel. 65637.   I thought I read somewhere that you need to update to 1.4.0 from the Utility before the App is able to handle the new firmware updates.  
1.  I have Model: Deco M9 Plus (US) Ver. 2.8.  
2.  Bought this from Costco online in mid July 2020.  
3.  I am using Windows 10 with the latest updates and did this from my wireless network.
4.  From my iOS App's Advanced page: IPv4 = Dynamic IP, SIP ALG = Enabled, UPNP=Enabled, Fast Roaming=Disabled, Beamforming=Enabled, Operating Mode=Router, Notifications=Enabled.  
5.  Note on "Fast Roaming":  I was not able to join Amazon Cloud Cams to my network because the App continually would not accept my login credentials (sound familiar?).  This was very frustrating until I contacted TP-Link support and they indicated some network components will not funtion properly with Fast Roaming Enabled.  Once I Disabled Fast Roaming, I was able to log the Cloud Cams onto the network.  Now I live without Fast Roaming. 

I had the same problems with the login credentials not being accepted.  After numerous failed attempts, I used an old latop running a 32 bit version of windows 10.  I was able to successfully login to the M9s and upgrade them to the latest firmware. 

Wow. Having the same problem with an "incorrect" username and password combination, I broke out an old 32-bit laptop and tried running the firmware updater utility from there.  Still no good...  Scratched my head for a while, then noticed in the iPhone app my username/email address had capitalization--e.g., John@gmail.com.  When running the utility I had been using john@gmail.com.  Capitalized that J and it worked fine.  OMG.  Terrible.  Just, OMG.

@FA44  Thanks for the information! 

I validate that these instructions provided by @FA44 solve the upgrading problem. After you upgrade the firmware, you can access the web base app. 

It's unbelivable that the support team doesn't know this! And to make things spicer, they decided to add a new firmware version which is labeled v2, but has a version 1 file inside!!!

I hope this product worth it, because Im definately not recommend this due to the support received.

Thanks again @FA44