What to do if new devices won't connect to Wi-Fi?

Question

WiFi

Given:

• Router D-Link DiR-825AC
  
• 5 serviceable devices based on various operating systems (android, windows, linux)

The problem is this:
The above router was bought six months ago, stood at home, and during this time worked properly. But about a month ago, problems began to arise with connecting devices that had never connected to the network they were distributing. That is: smartphones and computers that previously connected to the router continued to do so, and new devices (of which there are five) could not connect.
It becomes clear that the problem is in the wi-fi router itself, what should I do?
PS
Full reset and installation of a new network did not help.
Black\White lists are not set.
There is no limit on the number of users.