To use a 1Gbps ethernet connection:

a) The adapter in the computer needs to be able ro run 1Gbps

b) The cable connecting the computer to the router also needs to be able to run 1Gpbs. To do this all eight pins need to be connected rather than just 4. I suggest that you try a different cable.

In Windows 10 go to Control Panel, All Control Panel Items, Network Connections
Right click the Ethernet adapter
Select Status to display the connected speed.

Thanks Michael

My ethernet adapter (named unidentified network) is showing
No internet access
No network access
Speed 100 Mbs

My wifi adapter (named Nokiaxxxxxx) is showing
internet access
no network access
Speed 433Mbs

Does this mean that I have been connected to the Point of Access by Wifi rather than cable?
Sorry to be dumb - I just trusted the installer to get on with it.

Do you have a separate ONT to which the incoming fibre cable is attached?

If so can you run an etherent cable from your PC to the ONT?

I believe this is already happening. There are two connections from the Point of Access.
Cable 1 is a Cat 5E cable which goes to the WAN socket on the router.
Cable 2, a thinner cable, which I guess is a power lead runs to the nearest power socket where it is plugged into another box (ONT??) from which a Cat 5 cable runs to the PC.

In reply to a post by nb2017:

Does this mean that I have been connected to the Point of Access by Wifi rather than cable?

Unless you run a VPN. Best to check for Ethernet cables from the PC.

Unfortunately, I'm out of my depth here.
Could someone just tell me whether, if I were to connect a USB 3.0 to 1000 Mbps Gigabit Network Adapter between the first router and pc, that would "allow" me to get a speed more like that 900Gbps onto my pc.

In reply to a post by nb2017:

Could someone just tell me whether, if I were to connect a USB 3.0 to 1000 Mbps Gigabit Network Adapter between the first router and pc, that would "allow" me to get a speed more like that 900Gbps onto my pc.

Yes, but this would be megabits per second (Mbps) not gigabits per second (Gbps). A USB 3 to ethernet adaptor is workable, but not the best, unless you have no ethernet connector on your PC already.

Like this:
https://i.ytimg.com/vi/Mximhi1H8u8/maxresdefault.jpg

Sorry, getting megs and gigs mixed up.
Yes, I do have an ethernet connector available on my pc but AIUI that connects to the network adaptor which is only rated at 100Mbps whereas I need 1 Gbps for my new fibre connection and the USB 3.0 adapter seems the best way out.

In reply to a post by nb2017:

Sorry, getting megs and gigs mixed up.
Yes, I do have an ethernet connector available on my pc but AIUI that connects to the network adaptor which is only rated at 100Mbps whereas I need 1 Gbps for my new fibre connection and the USB 3.0 adapter seems the best way out.

Ok, the challenge with USB ethernet adaptors is they can take a lot of CPU, so if you are using the PC for high framerate gaming you may want to investigate if your PC can take an updated internal card instead.

Well, I've connected everything up and still cannot get the pc to regonise the connection - just says unidentified network.
The way the installation was left is a cat5e cable from the Point of Access to the WAN port of the router.
The power lead plugs in through a powerline adapter. This powerline adapter has one port to which was connected a cat5 cable which went into the pc. (I replaced that with a cat5e in case that was the problem).
But attaching the USB3.0 Gigabit adapter doesn't make any difference although the LED says it's working correctly.
I will look into another internal card but as it's an all-in-one that might be problematical. Incidentally, the spec that I have seen suggests a 100/1000/1GB capability anyway but will need to confirm that with the manufacturer.
Meanwhile, anything else I can try?
Many thanks for your help.