I use Netgear "range extenders". (after trying to re-use an old Belkin router to extend a modern Netgear router)
Just plug them in, somewhere in WiFi range of your router, and it creates a 2nd "cell" immediately.
The only drawback is that it's a different WiFi network so you can NOT just roam around the house and pick it up. You need to connect to the 2nd "cell" as and when it becomes the strongest signal. What you see on your "Available networks" for example is:
METCALFE WIFI NETWORK - Sig strength 3/10 CONNECTED
METCALFE WIFI NETWORK_EXT - Sig strength 9/10 (it auto creates the 2nd _EXT network based on the name of you existing network)
So, even if you are sat in the middle of a 9/10 extender signal, you may still be connected to your 3/10 router network.
I assume that a 2nd plug/adapter would create a 3rd "cell" with a different network name.
The nice thing about them is that, when I work in the garage, I can simply unplug the extender (normally on the upstairs landing, for bedroom coverage) and plug it into a kitchen socket. 30 seconds later I'll have a great signal (_EXT network) right up the garden and into the garage.