You should wait until your ISP-specific firmware is released. There have been reported problems with the retail firmware if your DLink Dir-850L is provided by TM or Time.

I've updated my router using that retail firmware, so far everything is good except cannot save client name & reserved ip for connected clients...

So, it's kinda annoying if you need/use reserved ip address in your LAN network but in exchange for added security, I can live without that for this time being...

Waiting for TM to release their certified firmware as well, hopefully everything will work normally...

Look at one of the comments :

You're forced to stick with the "@unifi" suffix after the upgrade. Even my original SSID "xxxxx@unifi" now becomes "xxxxx@unifi@unifi" lol

What da hell la

Too many wifi are using the same name with "@unifi" suffix like tm wifi (wifi@unifi), pppoe id, my nextdoor SSID....etc.
This suffix "@unifi" after successful upgraded, Is possible to change or rename or omit ?

wtf, i got firmware auto update last saturday without my knowledge, just check my panel why my firmware was change.

but lucky ssid didn't show the @unifi on wifi.

You means configuration file(backup file) or?

As when you upgrade firmware, it keep the configuration unchanged and bring over, so it is not applied to it.
Unless you apply new WiFi setting, which will add the @unifi when you saving it.

That's also why sometimes prefer to clear the config/factory reset to avoid issues.

Err....
When you upgrade to new firmware, there is no new settings being patch on your old settings, it just leave it as it be, and upgrade your firmware.
The @unifi that you saw on it is just a visual one, until you click the APPLY button, it will still be the old SSID without @unifi.

Unlike Windows 10, they import your old settings when there is big update like RS1, RS2, so the old settings will be patched by new default configuration, if they has written a script for it.