We've had this issue with a number of routers now; originally assumed it was the not very good router that Spark provided us with (Huawei dual band modem/router), but we now have a very good Netgear Nighthawk AX8 which has just started doing the same thing.  It'll drop connection for some devices but not others, usually starts with the mobile devices and then moves onto the rest.  It happened when we were on VDSL and is happening again now that we're on fibre, this tells me it's not the internal wiring of the house since the fibre doesn't use what was already here.  The wi-fi signal is still fine, but no connectivity.  Has anyone else had this problem and if so how did you fix it?   It seems to start happening about 6 months after a new router goes in.