Presumably when you say "a bookable guest room" the room is not actually your property and like wise the corridors leading from your apartment to the guest room
If that is the case it will obviously rule out running Ethernet cable from your apartment to the room and the use of powerline adaptors.
It limits what you can do.
You could try the following but because of how the discs connect to the BT Hub and depending on the distance and the thickness of the property walls it might not be an improvement on the signal that is already being received from your BT hub in guest room.
You could try moving your BT hub as close as possible within your property to the guest room or you could try getting one the following
BT Whole Home Wi-Fi | BT Shop
and place the disc into the guest room as and when needed.
If you have the BT Smarthub you could also consider getting the BT Complete Wifi package and again place a disc into the room as and when needed.
Complete Wifi | Wifi Extender & Booster | BT
If you have a BT Smarthub 2 and do not want to change your package, the BT shop sells the discs for both of the above systems.
