Issue with Samsung tablet and 21.02.1

I've just upgraded from 19.07.6 to 21.02.1 on a WRT1900ac(v1) and all seems fine except this Android tablet will not connect to wifi (it used to work fine). It first says "Connecting..." and then after a bit, "Authentication error occurred."
I'm not sure what the issue might be or how to troubleshoot it. I don't see anything in the system log or kernel log.
This only happens with the 2.4GHz radio. It seems to work ok with the 5GHz radio, but I'd really like to be able to use either. Numerous other devices all work fine with 2.4GHz, laptops, phones, printer, several Raspberry Pi.