Wifi is fast only for some devices. What’s the source of the problem? [closed]

        I have the Compal CH7465LG-LC router with UPC.

In the company we have 5 noteboos, 1 desktop, and several smartphones.

This router was working fine for years. It was configured with both 2,4Ghz and 5Ghz.
Since 1-2 months there are a lot of problems. at least 25% of the devices has slow wifi connection (5mb/s). And the rest has 400 mb/s.

I noticed that if I switch to only 5Ghz and I disable auto channel selection, and I put in manual, part of the noteboos that had slow connection, start to have a fast connection.
And the ones that has fast wifi, now have slow. The devices are all in the same room.

The same channel would work for few days. Then I have to re-search manually for a new channeel. But still I didn’t find a way to make all the devices work with fast internet as it was before, for years.

I am thinking that the modem has problems with the Access Point. It is not able to find the right channel.

Questions for you:
Is it normal that different devices work at different channels?
Should the router be able to communicate at different channels at the same time, or the router should choose only one channel that works best (good for all devices) and stick to this one channel?

I can’t understand what’s wrong.

Please help