Hello @Rubus 

Can you confirm or observe which channel the router is broadcasting as regards the 5GHz? I would suggest trying to modify the 5G settings on the A6, you can run a Wi-Fi analyzer on the phone to review the 5G networks around you, then choose one that with less interference with others to observe if the connection will be more stable.

BTW, is the A6 brand new or it was working fine before? You may also refer to this article to check if it is located in a proper location:

How to place your wireless router for optimal reception and performance?

What are the devices that connect to the 5GHz? Be sure the OS version or driver version for the wireless adapter is up-to-date. How about the 2.4G and wired connection, are they stable?

@Rubus 

As far as I can understand, it will not make a difference whether the internet connection bandwidth is 100Mbps or 1000Mbps. What you are experiencing seems only related to the 5G wireless connection. If you install a Wi-Fi analyzer on the phone and scan for the 5G networks, is channel 44 congested? If yes, please try changing to others on the A6 to avoid interference.

Since the router has been working for more than a year, you may try to reset it to factory defaults and reconfigure it from scratch to see if that makes a difference.