I've finally managed to fix the issues with 5ghz connectivity / driver issues on the T9UH / AC1900 USB dongle.

I used the instructions in this reddit post: 
- https://www.reddit.com/r/techsupport/comments/vleq0i/fix_realteks_usb_wifi_dongle_randomly/

plus this comment

 - https://www.reddit.com/r/techsupport/comments/vleq0i/fix_realteks_usb_wifi_dongle_randomly/nmuo4za/

The adapter was completely unusable before, but now it works with 5Ghz no issues so far.

I think I'm using the microsoft driver.