Do you intend to connect the old router(access point) to the new one FTTH router via LAN cable?
If so, then leave your old router just aside for now, and connect your computer to the new router, and open the Network and Sharing Center by going to your Windows OS desktop, on the bottom right, there should be a icon with a little computer with cable in the front.. Right click it, and click on "Open Network and Sharing Center",
You should now see a window that says "View your active network", with a connection type, whether WiFi or Wired, click on the blue text(link).
It should open a window that is the status of your connection type to your router. Click details.
Write down the IPv4 gateway, subnet mask, DHCP server, DNS server IP addresses. You will need that for later.
Now, power up your old router and reset it. There should be a reset hole/button at the back of it. Press the reset button until all the indicator lights of the router lights up, and let go of the button. The lights should go off, after a few seconds...
Unplug your FTTH router from your computer, and plug in the old router to your computer via LAN cable, I assume you have a LAN cable for that?
Open a new tab of your preferred browser, and type in the IP address located on the bottom of the old router. It should take you to the webfig. Login with the default credentials...
Set up your wireless, like the name and password, etc.
If you need to apply and reboot, do it, and login again.
Go click on Advanced tab at the top, find the DHCP settings link. Every router differ.
Then, after you found the DHCP settings page, disable the DHCP server. No apply and reboot yet.
There should be a option to set the Remote DHCP Server. Enable it, and type in the DHCP server IP you wrote down. Then go the Router IP boxes, located at the top, ish, type in the subnet mask first, usually 255.255.255.0
Your FTTH router have a IP, something like 192.168.X.X the first X is important. Type in at the Router IP box the prefix, which is 192.168.X then, choose another number between 1 and 253. Usually the router have the 192.168.X.1 as default IP, just use another number, like 2. Type in the last number you chose, usually 2. It should look like 192.168.1.2 for example. Don't forget the dots.
Check if the DHCP server IP is correct, along with the Router ip, to login later to.
Apply and reboot.
After it rebooted, power it off. Plug your old router into the FTTH router, via any LAN port. Connect your computer again to the FTTH router.
Try to connect with your phone to the old router. It should connect, and should provide you with internet.
You can now move the router to a place where it can extend the wireless network.
Give a holler if you get stuck, or have a problem.
