There are multiple solutions to your problem.
You can either resite the hub using Cat 5E (more flexible than Cat6 which is not really necessary) or by using powerline adapters between the ONT and hub.
Alternatively, you could extend the wifi by means of a mesh wifi system or again by the use of powerline adapters which use the mains wiring as pseudo Ethernet.
BT sell powerline adapters, but I regard the TP Link ones to be superior. There are many variations of adapters, some are just Ethernet which would be suitable for simply re-siting the hub or ones incorporating wifi.
Have a look here
Many thanks Liquorice, Very helpful indeed. I was initially thinking, after a I read your note, that the Powerline system might be the way to go. I've looked at the link you sent and I get the idea of one adapter connected, presumably fairly closely, to the hub by ethernet cable and then placing the other adapter on the far side of the house that is currently getting a weak signal, and that that would be connected by wi-fi. Do you think this would work if the second adapter is located just outside the house in an adjoining garage? i.e. through two or maybe three walls! We used to position the old hub right by the house door that was close to the garage, hence why I was thinking of a very long CAT cable to get the new hub in the same spot. So maybe I'd need CAT5E to be safer? What's confusing me is two things: what's a 'mesh Wi-Fi system'; and I don't get the idea you mention of using Powerline adapters to 're-site the hub', which is what I'd be doing with a long CAT cable. Sorry if I am being dumb. I'm very much a novice I'm afraid.
ONT to ethernet powerline adaptor via short ethernet cable. At other end Powerline ethernet adaptor to hub with short ethernet cable.
I think you are perhaps misunderstanding how powerline adapters work. They effectively turn your mains wiring into an Ethernet cable. You connect the master to your hub via an Ethernet cable and then the slave just plugs into any mains socket and provides either an Ethernet outlet or a WiFi hotspot or both. You can use multiple slaves with one master to provide WiFi and Ethernet ports.
You can resite the hub by simply using a pair of powerline adapters as an Ethernet cable.
A mesh WiFi system is simply a system of WiFi access points that are interconnected usually via WiFi. Probably easier to Google WiFi mesh rather than trying to explain it on here.
Thanks Liquorice, That makes things a lot clearer. It seems that I could get a couple of Powerline slaves and experiment as to what works best.
Thanks GaryC, Ok that’s food for thought too! I’d heard about BT disks but wasn’t sure about the strength of the signal via them. If there’s a consensus here among the different responses to my original query then it’s perhaps that careful use of CAT5E cable might be best (and probably cheapest).
If you simply wish to resite your hub in its original location and that gives you the WiFi coverage you require, then cable is the best and cheapest option followed by powerline adapters. Lag is only of concern if you are a gamer.
The whole home discs are a good solution if you want extensive WiFi coverage throughout the house.
Forgot to explain that if you use powerline adapters to resite the hub rather than using them as extenders with the hub remaining sited next to the ONT and connected by Ethernet lead, you won't be able to use them for wifi as well.