for wireless extension i.e no connection of cable between main and secondary router, a repeater or WDS router will do the job just fine,

the W8910G is unfortunately not supported by custom firmwares to enable repeating/WDS

if you're able to pull the LAN cable from main router to the first floor, then your TD-W8910G could be converted as an access point.

a basic guide here:

http://www.dslreports.com/faq/11233

GL

a typical standalone repeater, does provide one LAN port and you should be able to connect your second router to it if you need more ethernet ports. Otherwise, connect your PC straight to the repeater or get a cheap USB wifi adapter.

pros - no-brainer setup, simple and hassle free
cons - highly sensitive to positioning for signal strength with main router, only one LAN port, not flexible on network options, price


the option I use is WDS, with one TP-Link router upstairs on a 2nd wifi network.

pros - 4 LAN ports on a single power point, able to separate into 2 Wifi networks with different SSID, more flexible on positioning, stronger external antennae (upgradable as well), considered the more superior networking protocol to repeater, cheapest option
cons - WDS is non-standardised, it may or may not work with other branded routers, might be sensitive to positioning for signal strength with main router


you could also choose Homeplug option as mentioned by 'ihavenoidea', requiring two power points and two homeplugs one on each floor, and then connect a short LAN cable from the main router to the homeplug on groundfloor, then another LAN cable from homeplug on first floor, to your secondary router.

pros - best option when it works, packet loss between routers is reduced to almost nothing, streaming HD content and transferring files between 2 floors will be a joy
cons - most expensive option, takes up power points, might not work as good (or not at all) if both the power points are on a separate phase

all three options are perfectly fine on internet speed, surfing and downloading when they are successfully deployed.

GL