Hi @Martincz
That should be possible, yes. Any port can be the uplink port. Bear in mind, most service providers only provide a single IP address to use at your place of business or at home. This one IP address undergoes NAT to allow more than one device to connect to the internet at the same time. NAT is usually handled by a routing device, for example our GX20.
If the switch is directly connected to the internet and some other device uses the only available IP address from the service provider, the switch won't come online.
What do you connect the switch to on the other end of the SFP port?