@trigg3r yes, you need to buy a gigabit ethernet SFP transceiver.

Now, should you ? No, you shouldn't.

if the device you want to connect on the other side is less than 7 meters away. use a passive SFP DAC cable.

if < 15 m.. an active DAC cable

if farther. use a AOC (Active Optical Cable) 

if still want ethernet.. well.. you have to deal with the fact it is the most inneficient and potentially buggy way to do it, and remember random dude on the internet that told ya so.

Thak you  @crrodriguez! Now I'll have to read up on SFP DAC and AOC cable ... :-)

I have to cover a distance of 50 meters using an existing Cat.6 cable: how about if I use one of the 8 RJ45 ports on the switch instead of the SFP port?

This cable have two SFP+ connector: is it ok for the TL-SG3210 SFP (without +) port?

  @trigg3r both ends need - in general - to link at the same speed. while sfp+ devices usually work fine in sfp slots at 1gig.. I suggest you ask the vendor if they do gigabit just to be absolutely sure. (either automatically or force in either switch)

Of course you can just use N ethernet ports if you wish, that's the simplest solution.. I advise against ethernet transcievers when you already have other switch ports available.. why? they are of widely varing quality, some run scortching hot , have poor construction/software/choke full of bugs or are power hungry.

The more hardware,software is invovled the more the calls/bugs/time one has to deal with it. it is just have limited value, for "must be ethernet or drop dead" case IMHO.

  @crrodriguez thank you again!