I knew it was possible and it has taken me some time to suss this out. I'm not an expert and can't advise on anything other than my example - I will be connecting my old Plusnet router (Sagemcom - Hub One) to a Virgin broadband router. I have included a link to a YouTube video that was valuable to me, and it may help those that need further in-depth explanations.
This setup has increased my wifi significantly - from 15 Mbps to 48Mbps.
You will need a wifi extender socket with ethernet port. You will also need to reset your old unused router to it's factory defaults - refer to manual for instructions. On the Plusnet router it's just a case of pushing a small pin in to the reset slot at the back of the router.
You will need to find the old routers IP address, searching the web should make this easy. In my case I'm using an old Plusnet router, default IP address: 192.168.1.254
From your wifi menu/tab connect to the old router, this should show the default name if the reset has been completed correctly. You will need your original password, which should be on a sticker on your router.
Follow on screen instructions to enter MANUAL settings page. You may also be able to reach this easily by typing in the IP address manually.
Once you are in the settings you will be asked for the Admin setting, this can also be located on the sticker on the router.
From ADVANCED settings tab
select WIRELESS setting tab
Change router name and password and hit APPLY.
Your connection to the router will be lost.
Reconnect from wifi tab and enter the new password you just created. Follow same procedure to get back into MANUAL reset page.
From HOME NETWORK tab
select IP ADDRESSES tab
Disable DHCP
From FIREWALL tab
select CONFIGURATION tab
and select ALLOW ALL TRAFFIC
(Don't worry - your main router will still have it's firewall enabled)
Apply all changes and once again you will need to re-connect to router. At this point plug ethernet wire in to one of the ports - PLEASE NOTE - Do not use Broadband DSL or Broadband WAN ports.
Video link for further in-depth information : https://youtu.be/J5QJb3O19zI
