What do you mean cannot seem to get AC speeds or compatibility with 5G networks? What's the speed of your 5GHz? Did you try change the Wireless Channel and Channel Width of your router's 5GHz?

On your computer, go to Device Manager---Network Adapters, find the T2U, right click and go to Properties, then go Advanced, change the Channel like the picture below.

Windows 10 has the built-in driver for the adapter.

What's the operation system of your computer? Did you try change the Wireless Channel and Channel Width on your computer? Do your other wireless clients work well with 2.4GHz?

cunio wrote @Anton, thanks for the answer. I have already made this, but there are some issues with it: 1) http://tplinkwifi.net/ does not work properly, I have to write my IP to get to AP settings,...

The extenders have to in the same channel as the router's, that's why they can connect to each other wirelessly. When they are working properly on different SSIDs, you can try access to their setup...