It seems from this amazing thread that the best way to solve this issue is to stop using the Smart Hub 2 and use a different router instead.
That is not best advice in my opinion . My recommendation would be to add a wireless point to the SH2 and based on comments go for a non Bt brand.
You’ll need a wireless access point and I would place that in the room where coverage is poor or even in the loft. You will need a network cable to connect it to your smart hub. As a temporary solution and to test coverage you can use powerline connectors. Given your circumstances, it might be worth asking a local computer shop to supply and install it but it’s not difficult.