Hi
I have two identical DAP-2553s.  One is set for 2.4GHz, the other is set for 5GHz.  I have been having issues with 5Ghz stability for some time and after eliminating my particular unit as the cause, I tried rolling back the firmware to 1.12.  WOW!  5GHz stability once again!  Streaming connections that used to stay up for only a few minutes are now back to staying up for days!
One thing I noticed:  With 1.20, the signal strength of all my clients was 95% and higher.  When falling back to 1.12, they're all in the 70% range.  Again, this is only for 5GHz.
Also, 2.4Ghz performance appears flawless with 1.20.
I hope this is useful to someone.
Thanks