@Fotis_3D 

Hey

Just off the bat I notice your firmware is quite old, check here and download the latest 2020 firmware as it does have features for enhanced performance.

https://www.tp-link.com/uk/support/download/archer-vr900/#Firmware

Its likely not your ISP router, that would be unlikely to affect WiFi in any way...   However if you have 25 wifi devices its quite a bit for a home grade device, yes it can support 64 but really its not feasible.   Like cars saying they get 75mpg, you never will  :)

I would start with the update, get the firmware to the latest version.

Also try changing the wifi channels to manual and not AUTO, try 6 to start off with for 2.4ghz.     40 for 5ghz  

Do yo have your SSIDs seperated?  (different names)

@Philbert Thanks for the help.

I have manual channel 1 for 2.4ghz as I have tested it and searched that it is the best here and I have set it at 20mhz.

I do not use 5ghz but if I do it has different SSID as does and the guest network 2.4ghz.

To be honest I had checked through the router the firmware version and it didn't find any newer available, that's strange.

Also keep in mind that 18 of the wireless devices do not require so much bandwith. :)

@Fotis_3D 

Try a manual update of the firmware, sometimes it doesnt detect the update via the device itself.. had that on my old router

Thats likely you best option  

@Philbert I did it manually. Though I installed the latest that is beta but I will check how that goes for the next days.

Thanks. :)

Try a manual update of the firmware.

@Fotis_3D Problems continue. I added some more smart stuff and the router will not accept more than 23 devices connected.

It has become a real pain.