Power line

Extend the network via the power lines. If new cabling is not possible, using EoP (ethernet over powerline) may be a solution. In practice, this means that you plug a module into a socket, e.g. at your router and connect it to your router with a network cable. Thereby the network is sent via the power line to the place where they put the 2nd module. In module no. two, a network cable is also inserted, e.g. routed to a switch or an access point and voila - the network is extended.

The 2 (or more) sockets used for modules should be on the same phase. Your electrician can help you with this if you are in doubt.