Wifi disappears and does not communicate with devices randomly

[#1]

I had fibre connection installed a few weeks ago.  Had a standard ADSL by Plusnet until then. Since we have fibre, the wifi connectivity is really random.  The most affected are the various android devices (phones, tablets). Laptops seem to be better but they also experience random drop of wifi connectivity although not as often.
We have tried various solutions. sometimes, disconnecting and reconnecting the router helps, but not always.
Any advice?

[#2]

Assuming its the hub zero/2704n then logging into the router,  changing the wireless mode to up to 300mbps can help and you could experiment with different wireless channels if that doesn't do the trick.
Those would be the first checks to do

[#3]

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Any advice?

Definitely! First of all I'd get a WiFi scanner app installed on one of your Android devices, there's a few decent free ones out there.
Use that to scan and analyse the surrounding frequencies, this should also give you a clear idea of what channels those frequencies inhabit so that you can make an informed decision about which channels to try.