It could be the DHCP implementation on the Smart Hub being badly broken. There are numerous reports of this. The Smart Hub seems to remember all devices that have ever connected to it - you can see them listed in the admin interface. Old devices never get cleared out and after a while the system gets clogged up and has problems issuing IP addresses to devices wanting to connect - both via ethernet and Wi-Fi.
The only way to resolve the issue, at least until the list fills up again, is to do a factory reset. I've had this problem at a client site with a BT Smart Hub 2 which was recently installed hoping to solve the same problem on their previous Smart Hub. It didn't fix it.
I noticed the issue when I was testing wired connections around the house. I could plug in a network tester and it would get an IP address and connect to the internet. I could unplug it, plug it back into the same socket again and it wouldn't get an address even though there was a good physical link. I poked around on the BT support forums and found lots of posts about similar issues.
In the end I installed a firewall which acts as a DHCP server and all the connection problems have vanished completely. The client now has reliable internet for the first time since buying the house five years ago. Everything just works.
So, my suggestion would be to try doing a factory reset and seeing if this solves problems to start with. Just unplugging the router and rebooting it doesn't
clear the device list. Of course, the list will fill up again but this may give you longer periods of stable connectivity.
A longer term fix would require BT to fix the firmware or you could buy your own non-BT router.