I have recently upgraded to smart hub 2. However, neither our old cable (red) nor our new cable (yellow) fit. What do I do now
What are you trying to connect to what? Smart hub2 Ethernet sockets are exactly the same as any other router Ethernet sockets.
Do you have a white Openreach modem connected to your phone socket? If so, it is no longer needed just unplug the grey cable from the DSL socket on the Openreach modem and plug it in the grey DSL socket on the Smart Hub 2.
I am trying to connect to smart hub 2. I have a modem, but no grey cables only red and yellow ones that Bt have sent me, but don’t fit.
Does the modem plug into your phone socket? If it does, you no longer need it, the new hub replaces both the modem and the old hub. As posted previously, just disconnect the grey cable that is in the DSL socket of the modem and plug it into the DSL socket on the new hub.
If you have FTTP (full fibre to the premises) and you have an optical modem (ONT), then the red cable connects the LAN port on the ONT to the red WAN port on the hub.
Does your modem look like this?