I'm hoping this has an easy solution and not a warranty claim but: I have a BT "Smart" router. Been so-so up to now. Drops out occasionally but I put that down to crap BT networks. It really ought never to drop. Awful what one has to put up with.
Anyway, suddenly both my mobile phone and my tablet is claiming that the 2.4GHz wireless channel is unreliable and keeps disabling it for being a poor connection, whilst they can both use the 5GHz channel if I set them to use it. If I try to get it to use the 2.4GHz it takes forever obtaining the IP address. If it thinks it is connected, after just a second, it is back trying to get the IP address again.
They can't have both developed the same fault at the same time, it must be the router. Can;t see anything amiss on the admin page.
It's quite likely to be interference on the channel (within the 2.4Ghz band) that the router is using. That might be, for example, because a neighbour has a router operating on the same channel (or an adjacent one).
It might be time to try the 'reset' button on your router. (I'm always rather hesitant to do that, as I'm not sure exactly which settings are retained and which are deleted. However the key ones seem to have been kept on the few occasions that I've actually done it myself).
Located the reset button, but as it's working on 5GHz, incentive to deal with it hadn't been high. That stated, I had to reboot the router last night as neither phone nor tablet would connect. Not at home at present. Will give it a try when I'm back, thanks.