I've managed to get my Android devices to get working internet connections with the TL-MR6400, after reading this discussion - https://community.tp-link.com/en/home/forum/topic/606790.

Here's what I found:

• I've noticed that going to the "Advanced" tab and clicking on "Status", there was no IPv4 connection.
• Still in the 'Advanced" tab, I checked the dial-up settings for the connection for the default profile "KPN", and its "PDP type" was listed as "IPv4 & IPv6",
• I created a custom profile where I set the "PDP type" to "IPv4", and kept all other settings.

After switching to the new custom profile with IPv4, Android devices started to get a working internet connection.

  @rmam 

Thanks for your further feedback!

You can also refer to this official thread: Some client devices have no internet access such as Android devices or Chromecast etc.