I have a 802.11b/g/n/ac wifi router that (apparently) uses a Qualcomm chipset that I am also unable to connect to with my TSU9800.
It worked years ago, then a firmware update was released for the router and that was it.
No use talking to their support regarding issues with a device that had been discontinued for a decade (I tried, though...).

I assume it's an issue with the wifi chipset driver and/or (Qualcomm?) chipset firmware.

It looks like the UniFi 6 Pro also uses a Qualcomm chipset, so you may be running into the same issue.

I have a second (Linksys) access point with a Marvel Chipset that works fine with the TSU9800 - not sure about the 9600, but the 9800 also does 802.11g (54Mbit). Not that it makes much of a difference.

Sebastian