Hi.  Thanks for the warm welcome.  My DIR895L was capable of having the secondary 5GHZ at 300 / 250 (down/up)  on the 1.21 firmware (prior to the updated KRACK patch).  After, the upgrade to the KRACK patch, my secondary 5GHZ is weak weak signal are at 5 / 1 (down/up) speeds.  All other settings are left intact.  To further prove my point that the KRACK patch is issue, I rolled back to 1.21 (prior 2018 version) and I get the blazing 300 /250 (down / up) speed, SO I can conclude in my case there is some speed issue in the KRACK patch that is sowing down the secondary 5Ghz channel.  You have the same problem?    I already tried the factory reset and reconfig everything form fresh and same issue.   It's definitely not any other setting or cable.  I am using else the same, ONY updated the software.  Any thoughts?   
I am in the USA - N/A region.  I disabled SmartConnect.  2.4Ghz and 5.1Ghz (primary) is working, but speed is a bit lower, 5.1GHz (secondary) is the problem child after the 1.21 KRACK update.  The signal and speed is virtually non-existant; speed and signal is zero bars and speed is almost zero.   I am using 2 Samsung phones on the 5.1Ghz secondary channel.  I roleld back to 1.21 (2018 version) and get blazing signal & speed.  So, I can conclude in my case, it's the 1.21 KRACK patch.  Any ideas?  Has anyone raised with DLINK?