Additional Steps Tried:
- Changed the IPv4 DNS to 8.8.8.8/8.8.4.4, but it did not resolve the issue.
- Swapped the main Deco with a secondary one, but this also did not help.

Exact same issues with exact same firmware, but on X20 (1+2 extenders).

Did a quick remove of all units, which is how you factory reset.

Then set them up as a new installation. All works fine again.

I have a very simple setup and adding the same SSID with the same password skips the major annoyance of changing all of your devices. So it was less than an hour wasted including troubleshooting a bit first, but still not ideal.

  @kenji-wada 

beste, ik heb sinds de update problemen om diverse wifi apparaten die normaal gewoon verbonden waren verbonden te krijgen.

alle Deco's al verwijderd en weer toegevoegd. Router van kpn opnieuw gestart. Alles diverse malen geprobeerd echter probleem blijft bestaan.

Hi, thank you for your response.

- My internet service provider is OCN (Open Computer Network) in Japan。
- The current internet connection type on Deco is PPPoE.
- The main Deco is still showing a solid green light.
- I have tried restarting the ISP modem, but it did not resolve the issue.

As a conclusion, I was able to restore the Deco X50 by factory resetting it.

  @David-TP 

That's correct.

after updating to 1.5.5, some mobile clients kept disconnecting from Deco Wi-Fi?

My Deco X50 works in acces s point mode.

It concerns lots of different devices (inverter solar, airco, switches curtains, smartbike). 

much other devices just normaly connect.

Best regards

  @kenji-wada 

I have a mesh home network consisting of 4 Deco X55's which ! auto updated last night and I immediately had trouble with internet speeds. Tried rebooting them all through the app as well as hard reboots by disconnecting power to no avail. I have another backup router for another area of the house which was working fine,so I knew my ISP service was fine.

I Googled the issues and saw a number of people in same boat who did a factory reset of the all of their Deco satellites and said it fixed the problem. I didnt want to do that and saw one thing about making changes in the QoS settings and found that the "Total Bandwidth" was set to 0.1 mbps down and 0.1 mbps up, which is supposed to be total available bandwidth from your ISP. I simply changed both down and up to 1000 and it started working. 

Looks like the firmware update reset the QOS seetings to 0.1Mbps and changing this to 1000 fixed the issue for sure. 

  @kitofer, thank you so much, you saved my day. The total bandwidth reset to 0.1 Mbit is exactly the problem caused by the firmware update.